support for other Thunderbird and SeaMonkey

[doctorcolossus]

Stylish currently supports these other Mozilla programs.
How much extra work would it be to get User Style Manager working in them?

I'm a Thunderbird user and can help with testing and debugging. I don't use SeaMonkey yet. I actually just found out about it, but I'm interested in giving it a try soon. I did a quick search and found this tool which automates conversion of Firefox extensions to SeaMonkey, and here's a great-looking reference on the differences from Mozilla.