Here I put together some addons for Firefox, that proved to be useful in my experience:
by Geoff Smith
StumbleUpon makes it extremely easy to surf the best-reviewed sites on the web. It is by far the best collaborative surfing tool for browsing I've encountered.
Once loaded the add-on allows you to review and share great sites with like-minded people. Also, the program pre-loads websites so you don't waste much time waiting on new sites (you can change this setting). I have already found a ton of awesome sites using StumbleUpon.
by Mime Cuvalo
FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.
Along with transferring your files quickly and efficiently, FireFTP also includes more advanced features such as: directory comparison, syncing directories while navigating, SSL encryption, file hashing, and much more.
IE Tab allows you to embed Internet Explorer (IE) in tabs of Mozilla/Firefox.
This is a great tool for web developers given you can easily see how your webpage displays in IE, then tab back over to Firefox.
by Wladimir Palant
Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them. Right-click on a banner and choose "Adblock" from the context menu - the banner won't be downloaded again. Or click Adblock Plus icon in the toolbar to see all elements of the page and block the banners. You can use filters with wildcards or even regular expressions to block complete banner factories.
by Jacob Wright
MyStickies allows you to place sticky notes all over the web and organize them with tags. You can view, sort and edit your notes with their free web based tool: www.mystickies.com
I use this add-on everyday and all day. Sorting and listing all of your sticky notes in on place is awesome. Say goodbye to bookmarks.