TB opens but only to a shell (can't see mailboxes etc.). Selecting main menu items (e.g, TB>Preferences) produces no result (meaning, I can't open the preferences). I am not getting any error msgs. I have disabled TB access to Mac address book, also booted in safe mode (incl. from command line in terminal). I was previously running TB on Catalina. I have installed 91.0.3 three separate times on Big Sur. I suppose TB is trying and failing to find&load my old mailboxes etc. If I go to Get Msgs in main window, I can see my old accounts. Clicking on them produces no result though. If I go to the Message menu, I can drill down and see the pointers to local folders, but obviously can't access them (or actually move anything, as no inboxes appear). I can compose a new msg, but the From dropdown is empty and I cannot send. Attach does not work. In short, TB loads, but there is essentially nothing I can do with it. Any thoughts/help would be appreciated.