Looked through Firefox and Thunderbird extensions and have a setup I like now...
Some language ones:
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- EnigMail - OpenPGP message encryption and authentication
- QuickReply - Lets you reply in the mail window instead of opening a new one. Very nice, I've done some development in the past, still a few bugs
- ClearSearch - Adds back the Clear button for quick searching messages
- Show SMTP Username - this crashes thunderbird sometimes
- Mail Redirect - Allow to redirect (a.k.a "bounce") mail messages to other recipients