Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
Use the Evernote Web Clipper to save things you see on the web into your Evernote account.
Allows bidirectional access to Google Calendar
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
Access to Google contacts and synchronize them with Thunderbird address books.
Mozilla Firefox with Twitter instantly connects you with what's happening in the world. Search people and topics on Twitter using @ and # from the address bar.