Try to start *Windows* in 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

- XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Thanks Christ1 but what do I do after starting in safe mode?

Use Thunderbird. See if it behaves properly (or "better").

"All Mail" says to me that this is a gmail account. I think that you don't need to synchronize or subscribe to this particular folder. There's very little in All Mail that you won't also see in other folders.

Of course, not downloading All Mail isn't a fix for a real problem. But if you have a connection which is slow or has limited bandwidth, All Mail is something you don't really need.

Thanks Zenos. That does seem logical. I'll give it a try and let you know what happens. I have already tried compacting folders + re start with add-ons disabled + done various anti malware scans + disabled MSE anti virus and none of these made a difference.