Does it happen in safe/troubleshoot mode (hold Option when launching TB)? If it's OK in safe mode, I suspect hardware acceleration is enabled in normal mode (Preferences/General/Indexing). If that's the case, disable acceleration in normal mode and restart TB.

Thanks for your reply. Unfortunately the problem persists even in Safe Mode. When I clicked Make Changes and Restart / Disable all add-ons, it restarted with the message box completely blank. I tried again and the messages came up, with the same result when I tried to compose a message. I then tried it again, this time including resetting the toolbars, with the same result. Meanwhile, up came a notice that an update was available, so I installed that, but still no luck, plus this time the tab function doesn't work. And actually the update was for one I had already installed. I unchecked use hardware accelerator when possible, but the problem persists.

The other weirdness is that some of my various inboxes are displayed as tabs across the other open messages. Also, it completely rearranged how my mailboxes show in the pane on the left.

I don't have that hardware to test, but did you start with a new profile on this device or copy from another computer? Did you install from the thunderbird.net site? Did you actually run in safe mode? Clicking 'Make changes & restart' at the safe mode prompt will restart in normal mode. Just click 'Continue in safe mode', which will run with all add-ons disabled and a default layout, and acceleration disabled, anyway.

Just saw this.

Thanks for the ideas. I transferred everything directly from the old computer. I know I for sure restarted in Safe mode.

But did you click the continue in troubleshoot mode? or just click the reset buttons and exit that mode.

If it truly reproduces in troubleshoot mode (as Matt clarified) then screen shots of what you are seeing might help us better understand.