Hi!
First of all, the extension is great. However, I would like to mention lack of two (maybe three) features:
1. Set default encoding for writing messages. It may be because I use different language version for firefox. But my system locales are set to UTF, and still new message is always composed as ISO-8859-2. Could you add an opion for saving default locales?
2. IMAP folders. IDK if someone mentioned it, but it would be wonderful, if I could synchronize folders with my gmail account. I just swiched from Opera, where it worked just fine, but now your client doesn't recognize it.
3. I (don't know, this can be kinda tricky) Could you add PGP/GPG feature? Encryption and (which I lack most) signing messages? There was this Enigmail for Thunderbird, but it is obsolete now. You can get very tired of all this copy-paste operations