Interesting add-ons that I don't use or have not installed
Shows source comments for advanced application preferences in about:config
Send a tab or link to another device using Firefox Sync.
Use Windows and Internet Explorer Favorites directly from Firefox. No need to import, export or synchronize - the same Favorites appear...
Remembers last selected (hilited) extension/theme/plugin/userstyle in Add-ons Manager. Turn off/on the add-on selection list box and/or fine tune behavior via its Options.
When click on the selected tab, a history menu corresponding to it will display.
Adds a Google +1 Button to your urlbar to +1 any page. More on http://www.google.com/+1/button/
Screen Draw lets you draw or type text over the top of any page in any colour or size and then save it to png or jpeg.
Extract Text from HTML code and make the content more readable than code
Make your Firefox menu icon colorful instead of three boring lines.
Improvement User Script Manager Greasemonkey (based on version 0.922) - Configuration Functionality for User Scripts outside Profile Folder (e.g. in DropBox Folder to share scripts)
Multiple Addon Deactivator (M.A.D) is an addon that can (de)activate one, many or each addon at the same time
Dormancy 'retires' tabs that have gone unused for a while, freeing up that memory. It then restores the tabs to life when accessed.
EXPERIMENTAL. MAY EAT YOUR SESSION OR DO OTHER BAD THINGS.
Dynamic creation of bookmark-style toolbar buttons which you can customize.