Solved. Don't know why it's not possible to delete or mark posts as solved here...?

You can both delete, and mark a topic with a reply as solved. I assume the designers though if folks could fix the issue for themselves they would not go to the trouble of asking in a support forum.

Ah, yes I suppose. It's lucky I didn't delete this post actually. I managed to get it working and all my emails seemed to have been transferred fine and Thunderbird looked how it should but after having turned off the computer and come back to it now, when I open TB, the same thing happened as before with this strange grey broken TB. So I guess I really do need some help. Do you have any idea what might be causing this? Any help much appreciated.

This seems to be the same problem as this: https://support.mozilla.org/en-US/que.../1296794?page=2&utm_campaign=questions-reply&utm_source=notification&utm_medium=email#answer-1369866

I've tried the solution offered there which seems to have worked for now.