Firefox extensions are small add-ons that add new functionality to Mozilla applications. They can add anything from a toolbar button to a completely new feature. They allow the application to be customized to fit the personal needs of each user if they need additional features, while minimizing the size of the application itself (from the official Firefox Extensions site). Here are my picks for the Top Ten Best Firefox Extensions:
Adblock Plus - 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.
Video DownloadHelper - The easy way to download Web videos from hundreds of YouTube-like sites. This works also for audio and picture galleries.
Greasemonkey - Allows you to customize the way a webpage displays using small bits of JavaScript.
Web Developer - Adds a menu and a toolbar with various web developer tools.
Firebug - Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
Delicious Bookmarks - The Official Delicious Add-on seamlessly integrates your browser with del.icio.us, the leading social bookmarking service on the Web.
FlashGot - Download one link, selected links or all the links of a page together at the maximum speed with a single click, using the most popular, lightweight and reliable external download managers.
IE Tab - An extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox.
StumbleUpon - StumbleUpon discovers web sites based on your interests, learns what you like and brings you more.
wmlbrowser - Simulate WAP browsing by viewing WML (Wireless Markup Language) pages.
Gmail Manager - Allows you to manage multiple Gmail accounts and receive new mail notifications. Displays your account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.
And last, here is a couple more of my favorite Firefox extensions ONLY for Chinese users:
Access Flickr! - Bypasses the flickr.com filter in Iran, UAE, Saudi Arabia, China and other banned countries and places...
FoxyProxy - FoxyProxy is an advanced proxy management tool that completely replaces Firefox's limited proxying capabilities. It offers more features than SwitchProxy, ProxyButton, QuickProxy, xyzproxy, ProxyTex, TorButton, etc.