It's ok if it's an classic .tar.gz file like on ftp://ftp.mozilla.org , which I can decompress in /usr/lib and set a symlink into /usr/bin

Great, Thunderbird is my personal fave on my desktop machine. As lightning will be integrated into future versions of TB3, we will get everything I need
If Mozilla team recognize that TB is build and used on Maemo, they'll maybe do a version for 800x480