Under Thunderbird menu the options: About Thunderbird, Quit, Preferences, etc. won't open. Doesn't work in safe start mode with all add-ons disabled either. Tried installing downloaded app over existing app. Using ⌘, instead of menu didn't work either. The folder list display in Thunderbird reverted to default font size at the same time but the rest of the settings are still intact. This happened abruptly with no changes to the system. What to do? Thanks very much.

UPDATE: Finally figured out that not being able to access the Thunderbird drop down menu had to be a problem with the app. I dropped down to version 60.9.1 and everything works fine again. So, something in the update broke me. I won't update again for a while.