Add-on Converter for SeaMonkey is able to convert some
incompatible "Legacy" Firefox and Thunderbird addons into working SeaMonkey addons. See the
Extension Compatibility Tables for SeaMonkey
for a list of tested extensions and their compatibility with SeaMonkey.
Mozilla hosts an add-on site for SeaMonkey.
However, many add-ons previously available were also Firefox add-ons and have since been removed after Mozilla
decided to purge old Firefox Add-Ons from the site. You can install the
"Classic Add-ons Archive" which has archived old Firefox
Add-ons, many of these are SeaMonkey compatible or can be converted, be sure to refer to the compatiblity tables.
For Greasemonkey, OpenUserJS has a
port for SeaMonkey.
Alternatively the old Firefox version (Available on the archive) can be converted.
Other sources for extensions: Modified Firefox Extensions (now archived),
SeaMonkey.be Extensions list.
For a working Tab Unload Addon: Following instructions here you can use
Bartab Tycho, an addon made for Palemoon but works on Seamonkey if converted.
Themes and customization extensions are currently
on the SeaMonkey Add-Ons site.
If you want to apply custom CSS to the chrome,
here is a list of chrome URLs.
about:config settings I've found useful:
Show hide menu bar with alt - Set
Disable the middle mouse loading the clipboard in url bar - set
Official SeaMonkey Website
SeaMonkey Project Blog
How to install SeaMonkey on Ubuntu
SeaMonkey on mozillaZine forums