TorBridy should try both SOCKS port 9050 and 9150
TorBridy's SOCKS configuration should try both 9050 and 9150 and use the one that works.
You can crib Torbutton's local tor check here (which requires access to the control port): https://gitweb.torproject.org/torbutton.git/blob/master:/src/chrome/content/torbutton.js#l1701
We wrap that with the option to fall back to the remote check if transproxy is set here: https://gitweb.torproject.org/torbutton.git/blob/master:/src/chrome/content/torbutton.js#l1675
The remote check is here: https://gitweb.torproject.org/torbutton.git/blob/master:/src/chrome/content/torbutton.js#l1770
Unfortunately if you perform any network activity (including a remote tor check) with a socks port set, you may run into #8511 (moved), causing any hosts you contact to become unreachable after you change SOCKS settings. I am not sure if that caching behavior is present in Thunderbird too. The local tor check won't be affected by this bug, though.