I am on a MacBook Pro, 2017, with macOS Big Sur version 11.2.3. “Get info” tells me that Thunderbird is at 78.9.1. Address book—cannot edit contacts once entered; contacts will also not delete; to change contact information, another contact must be created. In the Thunderbird menu, “About Thunderbird”, “Preferences”, “Hide Thunderbird”, “Hide Others”, and “Show All” do not function. The “Quit” function works occasionally. Most of the time I go the dock, hold the icon until a menu forms, and select “quit” from there. The only way I can update Thunderbird is that when I get a warning that there is an update, I click on the warning. Going through "about Thunderbird" is not an option. What might be done to repair or replace Thunderbird? Which files from the “Library should I keep, and which can I toss? Restarting with Add-ons turned off changes nothing. Can anyone help?