Do the menus work properly in safe mode? Hold Shift when you launch TB, or Help/Restart with Add-ons Disabled. This will test, among other things, whether you have a broken layout (corrupted xulstore.json).

I held down the Shift key when starting the app, got the message saying it was now starting in safe mode, then I used it for a while (it was all ok - as expected, it's always ok right after launching it), left it alone, came back to it a few hours later, and the menus were gone again.

I found xulstore.json, copied its content to the clipboard and had it validated by jsonlint.com - which reports it's ok (at least syntactically).

Here's something that *might* be useful, though it's a bit of a stretch: If I click on the More button, the button gets depressed, but doesn't "pop back up" until I click elsewhere.

The down arrow (next to Get Messages or Tag) do the same thing - after I click on them, they remain in their depressed state until I click elsewhere. Same with the 3-line menu, I just realized.

I shut down and restart the app, everything's ok again.

I'd be happy to send any config files or logs if they exist somewhere. I only use Thunderbird for its RSS reader, so I shouldn't have any profile data or passwords embedded anywhere.

Can you run the VM in Windows safe mode and see if anything changes with TB?

As for xulstore.json, a default version is applied in TB safe mode, so if nothing changed, it suggests xulstore.json is not corrupted.

That was a great suggestion. I tried that, and left Thunderbird running for a few hours with the OS in safe mode. The menus kept working throughout, and I couldn't get TB to stop showing the menus (not that I have a reproducible way to get it in that state, other than leaving it running for a while).

I had forgotten that the Windows Classic mode is still available on Windows 7. I got thinking maybe this has to do with Aero... So I rebooted the OS (out of Safe mode), and changed the theme to Windows Classic. I left TB running for a few hours again, and try as I might, it could *not* get it to fail once more.

Perhaps it's just a coincidence, but "normally" the menus start failing well before the amount of time I left it running without Aero.

I can certainly try to revert back to Aero mode to see if the problem comes back. While this is a pretty strong clue, I'm not sure what that means in terms of what TB could do about it. If there's something else I should try, including trying out specially-created test builds, I'd happy to oblige.

I don't see why Aero mode would cause a problem, as it is a fairly common setup, but since it appears to work in W7 safe mode, which uses a basic graphics driver, I'd be inclined to update the driver, if possible, and see if it all works right in W7 normal mode with Aero. If not, turning off Aero is not a serious loss.

I think we can rule out Aero being the source of the problem: I left TB running overnight with Aero, and by the time I came back in the morning, the problem came back. I switched to the Classic theme on the fly (without restarting either the system, or TB itself). The problem was still present. I switched back to Aero, and (unsurprisingly) that was of no help either.

As for switching drivers--this is a Hyper-V VM, so the drivers that it's using are going to be as good as it gets.

Maybe I need to wait for a longer period of time back in safe mode to see whether it *really* gets rid of the problem, but this is a VM I use all the time, so I'm not sure how practical it's going to be for me to leave it running in safe mode for days.

FYI: I can also confirm at this point that it has nothing to do with the Windows theme in use. I've reverted to Windows Classic, and after letting Thunderbird run overnight, the problem has come back (right-click menus stop appearing).

TB is the only app I have that exhibits this problem, so even if it's some driver problem, it's difficult to say that TB is doing everything as best it could.