Bogus TBird error message on 64-bit Windows Vista

Thunderbird message:

Thunderbird is already running, but is not responding. To open a new window, you must first close the existing Thunderbird process, or restart your system.

At first, when I saw this message this morning, I thought: OMG, some Windows updates that I installed last Friday have actually gone and made TBird  incompatible with Vista 64! 

But then, as hinted from this forum post, this bogus error actually means that there’s something wrong with the user profile. In my case, the mail folder was sitting on a disk drive that was disconnected. I reconnected it and all was well as before.