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.
A very simple tabbed browsing extension that loads your homepage when you open a new tab. If you have multiple homepages, the first is chosen.
Status bar widgets and progress indicators for Firefox 4+
Visual bookmarks for Firefox. (Works with all latest versions of Firefox)
Translate the whole entire page or just a selection using Google Translate.
Screen capture Web Page, selected region or whole browser window, edit screenshots and save the images to Local File or Google Drive. Edit screenshots in a very user-friendly interface with the different tools.
AddThis for Firefox is the best add-on to make sharing and bookmarking simple. Have all your favorite web 2.0 social networking, bookmarking, blogging, and e-mail services at your fingertips. Share any page, anytime, with anyone.
Watch videos anywhere in cinema mode without leaving the website you're on.
Protects you from Phishing or Scams (through XSS injection attacks) while you're on Facebook™ - Version 4.2 introduces a Tracking Protection feature as well.
Fasterfox Lite: Performance and network tweaks for Firefox, without the Enhanced Prefetching. This is a fork from the original "Fasterfox".
— 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!
Allows editing of web page content while in Print Preview mode, prior to printing or saving as HTML or PDF. Compacts the layout and removes unwanted content such as adverts, sidebars and blank pages. Any element can be formatted, hidden or deleted.