- Download: FlashGot extension with Free Download Manager (though add-on supports many other download managers as well), and optionally Video DownloadHelper for downloading videos from sites like youtube. And Web2PDF Converter to save web pages as PDF.
- Bookmarks: Delicious (requires account on delicious.com)
- Anti-ad: I use the combination of Adblock Plus (advertisement and banner blocker) and Beef Taco (a cookie that blocks targeted advertisement from many companies, like Google, Microsoft or Yahoo).
- Development and Testing: Firebug and YSlow described on this page and Web Developer – a toolbar, which gives easy access to dozens of useful tools, from enabling and disabling different components, to CSS preview, to validation…
- Feeds, Blogs: I am not big fan of reading feeds, I prefer to go to the original site. However I still need to be notified when a new post appears on one of the sites I read regularly. Also I often have a bunch of “things I want to read later” (e.g. when I have time or when commuting). So I use Sage, which is a nice feed reader/notifier and Read It Later, which does what its name says.
- Boosts: I usually open many tabs, which bloats Firefox memory and CPU, I also like to install SaveMemory.
- Multi-functional: GreaseMonkey is a champion, with lots of existing scripts and ability to create new ones.
Installation links on Mozilla site: FlashGot / Video DownloadHelper / Web2PDF Converter / Delicious / AdBlock Plus / Beef Taco / Firebug / YSlow / Web Developer / Sage / Read It Later / SaveMemory / GreaseMonkey