Look these add-ons if not you dont know what you miss. Please vote my collection(!)
The easy way to download and convert Web videos from hundreds of YouTube-like sites.
This works also for audio and picture galleries.
The first and only download manager/accelerator built inside Firefox!
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
Displays a country flag depicting the location of the current website's server and provides a multitude of tools such as site safety checks, whois, translation, similar sites, validation, URL shortening, and more...
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager.
A customizable mouse gestures extension which enables to execute various commands and user scripts with six types of gestures.
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
The most beautiful yet the simplest add-on that makes a strong colorful appeal. Colors every tab in a different color and makes them easy to distinguish while beautifying the overall appearance of the interface. For Seamonkey & Flock. An essential. F...
AiOS lets you open various windows as sidebar panels, and quickly switch between them. So it put an end to the window chaos! In addition to bookmarks and history it opens dialogues such as downloads, add-ons and more in the sidebar.
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.
— Open web pages saved with Internet Explorer or other browsers (MHT)
— Save many tabs, video and audio reliably, in a single ZIP file (MAFF)
— Return to the original site you saved a page from, and more!