Addons that I am not prepared to put on my staples list, but I still use.
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).
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!
3D in your browser! FoxTab brings innovative 3D functionality to your Firefox.
New! Top Sites for FoxTab (aka Speed Dial).
Add a compact menu button to the navigation toolbar, and hide the menu bar.
Keep your favorite Extensions synced with all your computers.
This extension is a incremental search UI for Google Bookmarks(http://www.google.com/bookmarks/), You can find bookmarks very quickly.
Note from Mozilla: This add-on has been discontinued. Try Colorlicious instead.
Change Firefox to Any Color and create your own personalized theme.
Categorize combines advanced bookmarking and search with easy tab-management. It's a fresh approach! Say adios to confusing never-ending tags and complicated folders. Categorize organizes your web sites much faster and lets you access them in a snap.