Interesting add-ons that I don't use or have not installed
CSSsir is easy and handy tool which allows you to generate cross-browser code for main CSS- rules in a few clicks.
This CSS3 generator will be useful not only for web-masters and designers but also for common users.
Shows source comments for advanced application preferences in about:config
Send a tab or link to another device using Firefox Sync.
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.
Make your Firefox menu icon colorful instead of three boring lines.
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.
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)
Extract Text from HTML code and make the content more readable than code
Multiple Addon Deactivator (M.A.D) is an addon that can (de)activate one, many or each addon at the same time
When click on the selected tab, a history menu corresponding to it will display.
Dynamic creation of bookmark-style toolbar buttons which you can customize.