I have used plenty of add-ons, and browsed through even more than that. These are the ones that I recommend. These are the ones that actually make browsing the internet easier, faster, and safer.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Collector's Note Blocks most ads
Find out which websites you can trust. WOT adds intuitive traffic light-style icons next to search results and URLs to help you make informed decisions about whether to visit a site or not.
Collector's Note Shows a user ratings based rating on several privacy and security related categories of web sites. Recently shows ratings in searches and links of facebook and twitter.
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
Collector's Note Install pre-made skins for any web site. You can also write your own stuff. Careful... some of them change everything and can mess up your themes and personas.
LastPass, an award-winning password manager, saves your passwords and gives you secure access from every computer and mobile device.
Collector's Note Ultimate security by saving passwords over an encrypted network rather than locally. Even if someone should get access to your computer, your passwords are safe (assuming you set a password for this). Works for Internet Explorer and Chrome as well.
Xmarks is the #1 bookmarking and open tab add-on. Keep your bookmarks, and open tabs backed up and synchronized across computers and browsers. Mobile phones with Premium access. Utilize LastPass to sync passwords!
Collector's Note Syncs bookmarks and (optionally) passwords and open tabs between multiple computers/browsers. If you only use Firefox, I recommend just using the built in sync feature.
Adds a download progress bar and active downloads count to the Firefox icon on the launcher in Unity
Collector's Note Linux (using Unity) only- This shows download progress in the icon.
Don't like right-clicking? Use this restartless add-on to toggle your App Tabs with a double-click!
Collector's Note Double click a tab to make it an app tab. Doesn't save any clicking work, but still easy.
Mozilla Firefox with Twitter instantly connects you with what's happening in the world. Search people and topics on Twitter using @ and # from the address bar.
Collector's Note Quick and easy way to search people and topics on twitter. Just start off with either @ or #
Find pages from your history, on the web, for a particular category.
Collector's Note Combines the search bar and awesomebar into one. Unlike Chrome's way, this only sends what you search for, rather than everything you type into it. Does not yet suport custom search engines, but it covers pretty much everything. Also begins loading the search in the background for quick results.