Here have some extensions of the Firefox Web browser:
1. Flashblock: to block contens in Flash, for a faster navigation.
2. Firefox Sync: great extension to sincronize your bookmarks, tabs and passwords stored, in some computers.
3. Colorful Tabs: to organize your tabs by colors.
4. Echofon: good client for Twitter.
5. Easy YouTube Video Downloader: an imprescindible extension to download the content of a video in YouTube.
6. BugMeNot: allows the access to service that require a registration, without the process of registration (good tool for lazy people).
7. Autocopy: automatic copy of a selected text.
8. Google Translate: very usefull extentsion, to translate to some language a Web page.
9. Force-TLS: to garantize our privacity in the navigation.
10. Web of Trust (WOT): this extension show the grade of confidence that have the Web pages as the result of search.
My favorite extension is Firefox Sync, so I can take my bookmarks and passwords wherever you go, I find it very useful. Firefox has become an essential tool in my life, and yes it is FLOSS, which also allows me to install it anywhere without worrying about licensing issues.
For more information of these extension click in the reference link.