Hi! I just found Your wonderful addon, and now I'm able to use firefox similar as opera. Thanks!
When using compose window, its not possible to write in html, all buttons are inactive. I installed the offered beta-version and that was okay with html. But incoming mails couldn't be received. So I turned back to the 2.82 version hoping that html will be active in the next version.
I'm using FF 12, but also tried FF 14 before together with simple mail.