Installation of new version or update to thunderbird (cannot remember which it was) required a reboot to complete. At the same time Windows had announced a revision of windows 10 which was going to run automatically later during the night. When I went to restart the computer to complete the update it asked whether I wanted to restart or restart with updates. Unfortunately chose the latter. Since then have uninstalled the windows update so presumably back to version running prior to Friday the 13th but thunderbird still frozen in the "needs to reboot" mode. Problem solved. Turns out deleting Thunderbird from the apps folder in file explorer did the trick.