These extensions are for the advanced users and users working fast!
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Google translate gadget for firefox. This extension translates entire webpages into a language of your choice with one click. Idea comes from Google Translation Bar for Chrome.
Shows source comments for advanced application preferences in about:config
Provides CPU usage and memory usage graphs on the toolbar.
Note: Not compatible with Firefox 25+ (try FindBar Tweak instead). In Firefox 24 and below, adds a find bar button that allows you to search for a word/phrase on a web page and lists the results. Also displays the total number of matches found.