Restarted FF via Alt-F2 prompt instead (then entering "firefox -P Small --no-remote --safe-mode"), and I still can not open links from Thunderbird. I get: Firefox is already running, but is not responding. To use Firefox, you must first close the existing Firefox process, restart your device, or use a different profile. The fact that disabling all add-ons one by one does not affect the tabs not working behavior, but safe mode does, maybe indicates that it is not an addon, but a theme the culprit.