I have been ignoring Firefox extensions for ages, but I have finally given in and installed 14 plug-ins. Take a look at this article, which details most of the ones I installed.
Adblock Plus - removes ads
ColorfulTabs - they can't spell colour, but it's nice to have colours on your tabs
Ctrl-Tabs - I can move through my masses of tabs using Ctrl-Tab
DownloadHelper - for getting videos and images from sites
FireGestures - great when you get used to it although I find keyboard shortcuts just as good most of the time
Flashblock - gets rid of those annoying flash animations, click play to run them
Foxmarks - sync bookmarks over multiple computers
GooglePreview - get little preview window next to google search results
ScribeFire - for blogging, a bit clucky but it works
StumbleUpon - not really sure about this one
TimeTracker - records browsing time, a bit scary if you're online a lot
There's even one for making Internet Explorer a tab in Firefox....not sure why...I doubt it works on Ubuntu anyway.