these r the only links u will need for firefox extensions and themes
http://www.extensionsmirror.nl/index.php?showforum=2 <-- for extensions (over 500 of them)
http://www.extensionsmirror.nl/index.php?showforum=24 <-- for themes
for thunderbird
http://www.extensionsmirror.nl/index.php?showforum=3 <-- for extensions
http://www.extensionsmirror.nl/index.php?showforum=25 <-- for themes
bookmark 'em now.
and oh yeah, here's the Official Mozilla page listing all of the extensions on their site.
http://update.mozilla.org/extensions...&submit=Update
(rajesh's site listed only download-tool extensions)