This is a list of the most useful FireFox extensions I have Found.
1. Adblock Plus - So often now I go to a page I've found on someone else's computer to show them a cool site I've found; only to be hit with half a dozen banner ads that I didn't know existed. Adblock Plus is the best Plugin I've found. Hands down. It blocks everything you don't want to see (and, research has show you don't look at anyway).
2. Bookmark Duplicate Detector - Warns you when you are about to bookmark a website you've already bookmarked.
3. BugMeNot - Utilizes hundreds and thousands of user-submitted logins so you don't have to give your email address to every random blog you want to comment on. Or lets you comment without going through the lengthy signup process.
4. CustomizeGoogle - Make Google (and its children) look how you want them to.
5. DownloadHelper, Video Download and VideoDownloader - Various download services that simply fit into your right-click menu and let you download basically whatever you want. ie. Youtube videos.
6. Draganddropupload - Instead of locating the file you want in some random folder somewhere on your computer, just drag it to the file name text box.
7. Google Gears - Lets you download various Google eliminates. Such as your unread Google Reader posts.
8. Greasemonkey - Don't like it? There's a script. Want it changed? There's a script. Wish it could do this or display that? There's a script. No Script? Write one. Greasemonkey scripts allow you to do and have just about any functionality you may want, for anything.
9. Mr Uptime - I didn't think I would use this plugin at all. Boy was I wrong! Anytime I get to a site that can't be displayed, or is down for maintenance, or says to check back later. I use Mr Uptime. It checks the site for me until I can see it again and then it automatically opens the page for me. I use it all the time.
10. StumbleUpon - The best time waster in existence (except maybe the Yo Yo); StumbleUpon allows you to discover new worlds you didn't know existed at the click of a button. Really!
You may never have a use for these, but it can't hurt to try. They work for me so they might work for you.