Since you are new to firefox, I'd suggest you these extensions (besides Adblock Plus):
DownThemAll <-- this will give you a good download manager for firefox, with resume capability (If you are using a download manager like Flashget, then you should use the flashgot extension).
Nuke Anything Enhanced: This will allow you to remove any image, text or frame, that might not be blocked by Adblock.
BBCodeXtra: This adds items to the context menu that make your life in a forum really comfortable.
Paste and Go 2: This is useful when you want to copy links and paste it in the addressbar, or copy a word and look up in the search bar, just like in Opera
Greasemonkey: Install this extension and head over to userscripts.org and install scripts for specific sites to make your web experience a lot better. (you will find many userscripts for sites like google, orkut, amazon, etc. For example, there are scripts by which you can change the look and feel of sites like orkut and gmail)
QuickNote: Useful when you want to copy something like an article or a link and you don't want to save it in some file.
Google Browser Sync: Really handy extension. It saves your session, bookmarks, history and even cookies on google servers and the next time you connect to the net, you will have your previous sessions restored. It almost gives you your own Firefox workspace even when you connect to the net even on other PCs. You just have to give your password. The next time you format your PC, just install GBS again and type in your password, and all your bookmarks will be restored. (BTW, if you just want to save your bookmarks online, without any other stuff, I would say that you use Foxmarks Bookmark Synchronizer)
Other extensions that I would recommend you to try are:
Tab Catalog: Gives you a preview of all the tabs
Sage: News reader
Foxytunes: Control any media player from the Firefox window
FoxyProxy: (to be used with Tor) Browse through proxies in Firefox, the next time you can't access sites like Megaupload.
PicLens: 3d image browsing on Google Image Search, Flickr, Photobucket, deviantart, etc
NoScript: Disables Java, javascript and other scripts and allows you to run them only on sites you want.
That's all I can say for now. You might want to check out the most popular and recommended addons on addons.mozilla.org for some more
BTW, you might also like to add a few search engines in firefox. Head over to *addons.mozilla.org/en-US/firefox/browse/type:4/cat:all?sort=name or *mycroft.mozdev.org/install.html
