http://update.mozilla.org/
This Is the main site for Firefox 0.9 and 0.9.1 along with some for Mozilla and Thunderbird. It was abit rushed up for Firefox 0.9 release at last and is a bit of a hassle for themers/extension authors to get there stuff listed and updated as alot of that is done in bugzilla.mozilla.org. They do plan on allowing authors to have abillity to upload and manage their stuff in future.
http://extensionroom.mozdev.org/ is my favorite site for Extensions. Not all are for recent Firefox version(s) though they may still work along with the use of "Show Old Extensions" extension, or they may break firefox instead.
Firefox Help at
http://texturizer.net/firefox/ now links to update.mozilla.org for extensions and themes.
http://texturizer.net/firefox/keyboard.html
http://texturizer.net/firefox/mouse.html
For plugins like flash:
http://plugindoc.mozdev.org/windows.html