I have the same problem and also the apparmor DENY messages not only for vendor in /sys/devices/pci0000:00/0000:00:0x.0/ (also device and subsystem_vendor (when writing special apparmor filter for that). After setting
/sys/devices/pci[0-9]*/**/* r, in /etc/apparmor.d/local/usr.bin.thunderbird it crashes just with ExceptionHandler::GenerateDump cloned child blabla ExceptionHandler::WaitForContinueSignal waiting for continue signal... ExceptionHandler::SendContinueSignalToChild sent continue signal to child and without any additional apparmor dmesg warning. When stracing the whole stuff SIGTERMS when threading into /usr/lib/thunderbird/thunderbird Hope this will help and sorry no additional information for now (except: firejail thunderbird has other problems, next bug after debugging the apparmor stuff)
