Everyone will have an opinion on this but here is my list, post yours as well.
Tabbrowser Preferences (https://addons.mozilla.org/extensions/moreinfo.php?id=158&application=firefox)
Adds extra features to the default tabbed browsing options such as setting new windows to be forced as tabs and hidding the tab bar when only one tab is present.
Tab X (https://addons.mozilla.org/extensions/moreinfo.php?id=785&application=firefox)
Because I had right clicking to close them.. I would rather just nuke them with a click of an X
SpellBound (http://spellbound.sourceforge.net/)
A very good spellcheck plugin
Download Statusbar (https://addons.mozilla.org/extensions/moreinfo.php?id=26&application=firefox)
I hate the default download system, this adds a nice progress bar at the bottom of the screen
Super DragAndGo (https://addons.mozilla.org/extensions/moreinfo.php?id=137&application=firefox)
Very handy, does two things
1. Take any text you highlight and just drag and drop it/ toss it. It will be sent to the built in seach window.. So if you have Google currently set it will do a Google search for that text.
2. Do the same for a text URL and it will take you there.
IE Tab (https://addons.mozilla.org/extensions/moreinfo.php?id=1419&application=firefox)
For thosee IE only pages
SessionSaver (https://addons.mozilla.org/extensions/moreinfo.php?id=436&application=firefox)
Close / crash / restart the browsers right where you left off.. Close a tab? Bring it right back
Adblock Plus (http://bene.sitesled.com/adblock.htm)
A little better than regular adblock.. lets you block more objects as well as setup whitelist items.
Nightly Tester Tools (https://addons.mozilla.org/extensions/moreinfo.php?id=958&application=firefox)
Mostly usefull because it adds the option to turn of the version check per extention you install.. Very usefull for those good extentions that don't get updated often.
Well those are all on my list! Sure makes browsing a lot easier.
Three other ones that I use are:
GMail Notifier
https://addons.mozilla.org/extensions/moreinfo.php?id=173&application=firefox
Foxy Tunes
https://addons.mozilla.org/extensions/moreinfo.php?id=219&application=firefox
PDF Download
https://addons.mozilla.org/extensions/moreinfo.php?id=636&application=firefox