The new major release of the all-in-one Internet suite is available for download now. Building on the same Mozilla platform as the newest Firefox release, it delivers the latest developments in web technologies such as HTML5, hardware acceleration and improved JavaScript speed.

SeaMonkey is available in 26 languages, for Windows, Mac OS X and Linux.

Most notably, this release features for the first time:

  • The Content Security Policy (CSP) 1.0 parser has been enabled.
  • The Mixed Content Blocker has been enabled, blocking insecure active content loads on HTTPS pages.
  • New mail alert information can be customized now (Preferences/Mail & Newsgroups/Notifications).
  • A confirmation prompt has been introduced to protect against accidental permanent data loss when force-deleting messages using Shift+Del.
  • MailNews Save As Template supports multiple selections now.
  • The size on disk is now shown for newsgroup folders.
  • Added support for scrollbar style in Mac OS X 10.7 and newer.
  • Enabled mixed content blocking to protects users from man-in-the-middle attacks and eavesdroppers on HTTPS pages.
  • Improved about:memory's functional UI.
  • Enabled DXVA2 on Windows Vista+ to accelerate H.264 video decoding.
  • Simplified interface for notifications of plugin installation.
  • Enabled users to switch the search provider across the entire browser.
  • CSP policies using the standard syntax and semantics will now be enforced.
  • Implemented the HTML5 form control.
  • Added unprefixed requestAnimationFrame.
  • Dropped blink effect from CSS rule text-decoration:blink and completely removed element.
  • Fixed several stability issues.

What's New

SeaMonkey 2.53.14 contains (among other changes) the following major changes relative to SeaMonkey 2.53.13:

  • Updates to the following DOM HTML element interfaces: Embed, Object, Anchor, Area, Button, Frame, Canvas, IFrame, Link, Image, MenuItem, TextArea, Source, Select, Option, Script and Html. Please test add-ons.
  • Continue the switch from Python 2 to Python 3 in the build system.
  • Add ESR 102 links to debugQA bug 1779028.
  • Remove about plugins from help menu bug 1779031.
  • Remove url whitelist from SeaMonkey bug 1779037.
  • Dead links in cs_nav_prefs_advanced.xhtml [en-US] bug 1783558.
  • Dead links in cs_nav_prefs_advanced.xhtml bug 1786030.
  • Remove obsolete chat services from SeaMonkey address book bug 1779034.
  • Address Book: "Get Map" button is not shown for home addresses bug 1779319.
  • Added compatibility for rust 1.63

SeaMonkey 2.53.14 contains (among other changes) the following major changes relative to SeaMonkey 2.49.5:

  • The Bookmarks Manager has switched its name to Library, and now also includes the History list. When History is invoked, the Library will be shown with the History list selected. The extensive modifications were needed because of Mozilla Gecko platform API changes.
  • Download Manager has been migrated to a new API. Although it looks pretty much the same as before, the search option is missing and some other minor details work differently. The previous downloads history is removed during the upgrade.
  • The layout panel was added to the CSS Grid tools.
  • TLS 1.3 is the default SSL version now.
  • Support for all NPAPI plugins like Flash, Java and Silverlight has been removed. For displaying pdf files in the browser you can use pdf.js-seamonkey from Isaac Schemm.
  • SeaMonkey now uses a new api for formatting regional data like time and date. Default is to use the application locale of the current SeaMonkey build. If you use a language pack or a different OS formatting this is usually not desired. You can change the formatting from the application locale to the regional settings locale (OS) in the preferences dialog under "Appearance".

SeaMonkey 2.53.14 uses the same backend as Firefox and contains the relevant Firefox 60.8 security fixes.

SeaMonkey 2.53.14 shares most parts of the mail and news code with Thunderbird. Please read the Thunderbird 60.8.0 release notes for specific security fixes in this release.

Additional important security fixes up to Current Firefox 91.11 and Thunderbird 91.11 ESR plus many enhancements have been backported. We will continue to enhance SeaMonkey security in subsequent 2.53.x beta and release versions as fast as we are able to.

SeaMonkey-specific changes

SeaMonkey now uses gtk3 on Linux. If you experience a problem because of this please file a bug and link it to bug 1367257. Please try another OS theme first. Some of them are buggy and cause problems with SeaMonkey, Thunderbird and Firefox.

System Requirements, Installation and Uninstallation

  • Before installing, make sure your computer meets the system requirements.
  • Please note that the new Windows x64 version will only support Windows 7 x64 and up. You need to uninstall a previous Windows x86 version if you want to use it. Installing the x64 over the old x86 is not supported and does not remove the old version which might cause problems later.
  • SeaMonkey 2.53.10 will no longer offer to migrate your data from SeaMonkey 1.x or Mozilla 1.x at the first start after installation (bug 689437). In order to upgrade from such an old version, install the last SeaMonkey 2.0 release first, do the one-time profile upgrade, uninstall SeaMonkey 2.0 and then install your target release (e.g. this one).
  • If you are on an older release we suggest upgrading to SeaMonkey 2.49.5 first.
  • You can find more details about installation, profile data and uninstallation in our install and uninstall document.

Extensions (Add-ons) and Themes

  • Extensions and Themes for SeaMonkey 2.53.10 can be installed from the SeaMonkey Add-Ons website or the "Get Add-Ons" section in the Add-ons Manager.
  • Extensions with binary components such as Enigmail only work for the specific platform they are released (e.g. win extension for Windows, linux64 for Linux 64 bits). Besides, matching version of extensions for SeaMonkey may not be the latest available of those versions. Therefore, they need to be adjusted for each new SeaMonkey release, which also means that an old version of such an add-on can break SeaMonkey if you disabled compatibility checking (which e.g. happens when you install the Add-on Compatibility Reporter add-on). Please, note the following warnings and advice regarding extensions:
  • Previous versions supported a proprietary JavaScript try/catch solution. This was removed in bug 1228841 and needed to be backported to 2.53.10b1. The backport might break some add-ons. One add-on affected is the classic version of DownloadThemAll. Please ask in the support groups if a new version is already available.
  • For Enigmail, please refer to the Enigmail page.
  • The latest supported version is 2.0.9 Enigmail but there have been some problems reported against it.
  • Adblock Plus 2.9.1 may cause huge memory and CPU consumption and is not recommended or officially supported in this SeaMonkey version. Consider switching to uBlock.
  • Lightbird 0.7.1. is not compatible with this version. The message preview pane will not update itself correctly.
  • The latest 1.16.x working version of uBlock can be found here. Please install the latest legacy version.
  • The latest working version of NoScript Classic 5.x can be found here.
  • Dictionaries in WebExtension formats do not work, even though they appear to install correctly. Support for them will be added at a later date. Please install the latest classic version from the dictionary history or unpack the dictionary inside the WebExtension package into your /dictionaries directory.
  • Lightweight themes are no longer provided on the SeaMonkey Add-ons site.
  • Full theme add-ons may need changes because of user interface and internal changes. If you find any problem with themes, contact the theme author. Before reporting a problem with the user interface please make sure to recreate it with either the Classic or Modern theme.
  • Because of security backports which removed obsolete code, Session Manager add-on version or above is required to be compatible with current and future 2.53 releases.

Known Issues

  • This list covers some of the known problems with SeaMonkey 2.53.10. Please read this before reporting any new bugs.
  • Some translations are missing in a few languages in the user interface and are being substituted with the corresponding US English terms. We plan to update the l10n translations for the final version but some languages like Polish are now missing a translator. Consider helping out with translations if you would like to have a 100% translated version.
  • Opening single new tabs from the Library always loads them in the foreground. The problem is tracked in bug 1619108
  • SeaMonkey will not start under Linux if you downloaded the wrong package.
  • We are providing x86 and x64 versions of SeaMonkey for Linux. For newer distributions you probably need the Linux x64 version.
  • Lightning or one of the distributed extensions is disabled under Add-ons.
  • Starting with SeaMonkey 2.53.7b1 the distributed extensions can no longer be removed. If you have previously enabled one then under some circumstance it might be disabled during the upgrade process. If the extension is disabled and can not be enabled then it is because an old or incompatible version is still present in your profile.
  • To clean out old versions use the following instructions:
  • Enter "about:config" in the location bar.
  • Enter "extensions.installedDistroAddon" in the search field.
  • Reset (delete) all keys found.
  • Open the Add-ons Manager. If the "remove" button is shown for the disabled extension use it. Stored data like your personal calendars will not be removed by this action.
  • Restart SeaMonkey.
  • Disable any distributed extension you do not wish to use.
  • Unable to connect to secure sites and a certificate error is shown.
  • In 99% of all cases this issue is caused by the same problem. Your antivirus software or custom firewall acts as a man in the middle and intercepts secure https traffic. In order to intercept and scan this encrypted network traffic, it normally uses a vendor specific certificate which is not installed by default in SeaMonkey by the third party software installer. If you want to keep this feature working you need to install one or more certificate(s) from the antivirus software in SeaMonkey. You need to ask your antivirus software vendor where they are located. Consult the SeaMonkey online help to find out how you can import and manage certificates.
  • If you experience a hang and/or get a message box warning that a script is busy when opening the Data Manager try deleting the webappsstore.* files from your profile folder when SeaMonkey is not running. These files contain data stored by websites and will be recreated during the next start. If you are unsure if the information is needed please back up the files before deleting them. The problem is tracked in bug 1305624 but so far no coding error has been found. This problem has been reported frequently when trying to manage passwords. But the problem will generally happen just after you open the Data Manager, because this component is also used for managing cookies, site preferences permissions and other settings.
  • If you are running Windows 7 "Send to > Mail recipient" might not work. More information can be found in bug 1356902. As a workaround install the "Microsoft Visual Studio 2015 to 2019" redistributable runtime library. For x86 install the VS2019 x86 runtime.
  • If you are using the x64 build install the VS2019 x64 runtime. SeaMonkey was built using the latest VS2019 compiler so make sure to upgrade any older VS2017 runtime.
  • If you experience a delay showing the first page after start, please delete the Safebrowsing folder in your profile. Bad Safebrowsing data may delay showing the first page for up to 30 seconds. The problem is tracked in bug 1432274. This can occur even if you are not using Safebrowsing because the lists are always fetched.
  • SeaMonkey does not currently support the WebExtensions add-on api. Some popular add-ons like NoScript and uBlock Origin are no longer shown because of this on the SeaMonkey add-ons website. You can usually get compatible versions from the manufacturers site. WebExtensions support is a planned feature for 2.57. For dictionaries please install the latest non webext version.
  • The SSL 3.0 encryption protocol is no longer supported due to security concerns (bug 1106470). Legacy websites may still rely on it and will not work with the newer TLS 1.x protocols. Contact the website administrator to update their server to resolve the issue at the source.
  • The Remember Passwords add-on breaks password handling. Please disable or uninstall this add-on using the Add-ons Manager.
  • You cannot set up a new Sync account or pair a device anymore (bug 998807).
  • Data loss warning: If you use a profile with this or any later version and then try to go back to SeaMonkey 2.49.5 or earlier, SeaMonkey will rename your bookmarks and history file to places.sqlite.corrupt and create a new places.sqlite file, effectively resetting your browsing history. This is because the internal format changed and the upgrade process only works in one direction. Additionally, running MailNews with local folders and POP3 mail filters may produce summary files that are not correctly read by previous versions of SeaMonkey. If you decide to go back to a previous version of SeaMonkey, you should do it from a profile backup only. Stored passwords and local storage will also be unavailable in the old version.
  • After creating a news account through clicking a news URL, the MailNews account wizard may be broken (the account type will always be News). To work around the problem, set the mail.server.serverX.valid pref (where X is the internal number of the corresponding news account) to true in about:config, or delete the invalid news account (bug 521861).
  • WebRTC has been disabled by default. It needs an add-on to enable it per site. See the webrtc-permissions-ui-toggle add-on.
  • Web pages may appear differently in the MailNews feed reader than in the browser (bug 662907). Workaround: Add a "noscript {display:none}" rule to your userContent.css (note that the Stylish add-on cannot be used here since it does not integrate with the MailNews feed reader).
  • Lightning keyboard shortcuts for accessing the calendar tab from within the MailNews window or from the main menu (Ctrl+Shift+C/D) do not work. (bug 514512).
  • If you try to start SeaMonkey using a locked profile, it will crash (see bug 573369).
  • In ChatZilla users are unable to add an exception for an invalid certificate unless an override is added to the preferences (see bug 1738597).


  • SeaMonkey 2.53.7b1 can not be automatically updated to later beta versions as it is set to the wrong update channel. To overcome this, make sure SeaMonkey 2.53.7b1 is not running, edit the channel-prefs.js file located in the defaults/pref directory of the installation directory for SeaMonkey 2.53.7b1 and change the pref from "release" to "beta". When SeaMonkey is restarted it should then find future beta releases.


  • You might not be able to install SeaMonkey in Windows 10 if you have turned on the Windows Defender Smartscreen feature. Your SeaMonkey download will be blocked in the case. To overcome this go to "Windows Security->App & browser control" in the new Windows Settings and set the "Check apps and files" setting to Warn from Block.
  • Antivirus scanners may report that some parts of SeaMonkey (e.g. the file freebl3.dll) are suspicious. If you downloaded SeaMonkey from one of the official download sites, this is a false alarm. You might experience problems with secure websites when this happens. To fix the issue, instruct your antivirus software to ignore these files (and move them out of quarantine) and/or switch to another antivirus software and reinstall SeaMonkey.


  • The scrollbar is not always visible on some websites under recent macOS versions (10.15 upwards) see bug 1684651.
  • You might not be able to install SeaMonkey under recent macOS versions. In this case please read Open a Mac app from an unidentified developer for a solution.
  • When hardware acceleration is off, background colors in the UI (toolbar, status bar etc.) might not update correctly when you switch focus from/to a window. Users running Macs with certain older hardware will experience this even if they haven't turned off hardware acceleration in Preferences (Appearance/Content) (bug 626096).
  • SeaMonkey 2.53.7b1 can not be automatically updated to later beta versions as it is set to the wrong update channel. To overcome this, make sure SeaMonkey 2.53.7b1 is not running, edit the channel-prefs.js file located in the Contents/Resources/defaults/pref folder of the installation folder for SeaMonkey 2.53.7b1 and change the pref from "release" to "beta". When SeaMonkey is restarted it should then find future beta releases.