Advanced about:config and user.js
Firefox's Options menu provides access to a decent selection of options, though there are more (much more) available. Perhaps the simplest method to access and modify these is to type about:config into the Firefox Address bar and press Enter. This displays a (nearly complete) list of preferences available.
The Filter field can be used to search for entries which contain the keyword entered, e.g. cache, while pressing Show All will display all available preferences. Modifying the values for preferences is achieved double clicking or by right clicking on a preference and selecting Toggle (For true/false type options) or Modify (For options which accept other values/text) as appropriate. In the event the preference you are searching for is not listed, it can be created by right clicking and selecting New then String, Integer or Boolean as necessary (Enter the preference name first, then the value).
Changes to preferences displayed here are indicated in bold and stored in the prefs.js file (Which is located in the Profile directory). This file can be opened with any text editor and should appear similar to the beneath:
# Mozilla User Preferences
/* Do not edit this file.
*
* If you make changes to this file while the application is running,
* the changes will be overwritten when the application exits.
*
* To make a manual change to preferences, you can visit the URL about:config
* For more information, see https://www.mozilla.org/unix/customizing.html#prefs
*/
user_pref("accessibility.typeaheadfind.autostart", false);
user_pref("accessibility.typeaheadfind.enablesound", false);
user_pref("accessibility.typeaheadfind.flashBar", 0);
As per the warning, there's no need to edit this file if you intend to solely use about:config (and if it does become necessary for whatever reason, close all Firefox windows before doing so, otherwise changes made may be overwritten).
Alternatively, you can store custom preferences in the user.js file, this file must be created manually (A simple way to do this is to copy the prefs.js file, rename it to user.js and delete whatever is currently listed in it). Preferences entered here are copied into the prefs.js file whenever Firefox is loaded.
Preferences are entered in the same format as the prefs.js file, i.e. user_pref("preference", value); For example user_pref("accessibility.typeaheadfind.autostart", false); If you're into commentary you can add comments by entering // at the start of a line, for example:
//Controls caching of image/chrome data into RAM
//Default: true
user_pref("browser.cache.memory.enable", false);
It's important to note that if you delete any preferences that were stored in the user.js file, they will not be changed in the prefs.js file, e.g. if you had set user_pref("accessibility.typeaheadfind.autostart", false); and delete it from user.js several days later, that value will remain the same in prefs.js.
Regardless of which method you choose to use, some of the more relevant/interesting preferences available are as follows (Divided into appropriate sections). Note - For the purposes of creating entries the type (String, Integer, Boolean) is included in parenthesis after the preference name.