Summary: Extensions for Mozilla Firefox
See: FirefoxBrowser, MozillaFirefox, InternetExplorerAlternatives

Extension Sites

* Mozilla Update (offical Mozilla extensions site)
* The Extensions Mirror (popular alternative, more up to date and has a greater number of extensions than Mozilla Update)

Ad Blocking

* AdBlock (all you need for ad blocking really)
* AdBlock Plus (an enhanced version of AdBlock)

Web Development Extensions

* Aardvark (remove elements from a page, see how pages are created, view source code for parts of page)
* Add & Edit Cookies
* Checky (online validation for HTML/XHTML, CSS, RSS, XML etc)
* EditCSS (editing CSS is also party of the Web Developer Extension)
* IEView (right click a link to open the page in IE)
* LiveHTTPHeaders (view HTTP header sent to and received from a web server)
* User Agent Switcher (for those sites that still use outdated detection techniques and/or don't test in browsers other than Internet Explorer)
* View Cookies (view cookies in the Page Info dialog box)
* Web Developer Extension (a must install for web developers)

Social/Community orientated extensions

* BlogLines/LiveLines (subscribe to blogs when you sign up for a free Bloglines account)
* StumbleUpon (see what sites others like in areas you choose, and indicated to them what you do and don't like)

RSS/Atom Syndication

* BlogLines/LiveLines
* Feedview ('Feedview is a Firefox extension that shows newsfeeds/blogs in a more stylish way.')
* Habari Xenu (XUL news aggregator)
* infoRSS (shows feeds in the status bar)
* Lektora (RSS reader for Firefox and Internet Explorer, Ad Supported, or no ads for a fee)
* Sage (sidebar for Firefox)
* Wizz RSS News Reader (news aggregator that support enclosures/podcasts)