According to Mozilla's Justin Scott, Product Manager for Add-Ons, the average Firefox extension increases startup time by about 10%. The actual impact in seconds will depend on the hardware and software a user is running, but the company estimates that installing 10 add-ons will double the amount of time it takes the browser to launch.
To that end the company will begin running automated performance tests on the top 100 add-ons and post the results here on a weekly basis. The worst offenders currently include the FoxLingo Translator/Dictionary and Firebug website debugging tool, both of which slow down the browser's load time by 74%. The popular Xmarks ranks ninth with a 30% slowdown, and FastestFox, which promises to make browsing faster, will slow down Firefox 33% when launching.
Mozilla is contacting the developers of the worst performing add-ons to encourage them to improve their performance and will provide a performance-measuring tool for them to test against. The company says that developers should be aiming for a "5% start-up impact or less."
Within the next two weeks, Mozilla will add a warning to any add-on that slows Firefox's startup time by more than 25%, and in an upcoming version of the browser, third-party add-ons from standalone software vendors will not be installed unless the user specifically approves the installation (think browser toolbars and the like). This should have a significant impact on Firefox performance and give users back the control they should have over their add-ons, according to Scott.