Add-ons (both browser and mail) I've ever found interesting, most of which are SeaMonkey compatible.
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.
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
View HTTP headers of a page and while browsing.
Adds some tools to import and export folders and messages
Adds the ability to create and edit entries in the password manager.
Secure Login is an extension for Firefox integrated password manager.
Toggles dictionaries automatically or manually and displays the currently selected one
Offers the possibility to display documents in browser window.
Toggle word wrapping and PRE element wrapping in the message composition and HTML display windows, respectively.
Collapse and expand quotes. Please see the extension homepage for...
Preserve the modification timestamp, sent by the server, for downloaded files