Interesting add-ons that I don't use or have not installed
Shows source comments for advanced application preferences in about:config
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.
Simple content blocker, will block URLs based on the content of a simple text file in your profile directory (If you don't know the location of your Firefox profile, this is not the extension for you!)
When click on the selected tab, a history menu corresponding to it will display.
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.
Send a tab or link to another device using Firefox Sync.
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
Dynamic creation of bookmark-style toolbar buttons which you can customize.
Adds a Google +1 Button to your urlbar to +1 any page. More on http://www.google.com/+1/button/