Start *Windows'* safe mode with networking enabled - win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode - win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

Still In Windows safe mode, start thunderbird in safe mode - http://support.mozillamessaging.com/en-US/kb/safe-mode

Does problem go away?

Thank you for the response. The problem eventually cleared itself up.

paulbeam said

TB upgraded itself to 31.5.0 last night. This morning, it just had a spinning indicator up by the Inbox tab as it tried to connect/download from Gmail (it connected/downloaded from Yahoo and Comcast). Finally freed up after about 10 minutes and showed me some message headers. But after selecting any message, it doesn't show the content (unless it's one I've viewed previously) -- it just says "Loading Message..." for a LONG time, then that message goes away, then eventually (many minutes) shows the content of the message.