I would suggest this solution to another question where this happened:

• https://support.mozilla.org/en-US/que.../1053599

Many thanks. This works after a fashion, after trying maximise and size a few times I got control of it. First all I was only able to restore the one tab I was highlighting, then I got the whole set maximised but when I tried to windowise the session it disappeared again, then by doing it several times one clicking combination worked (not quite sure which).

So it would seem that in some way the window has resized to a single pixel rather than disappeared totally. Does anyone have any idea what's causing that?

I do not, but the xulstore.json stores this information. I would imagine if you open this file in a text editor you might see some dimensions and a status of the open windows. It is possible also to see if its an add on or extension by disabling each of them for process of elimination.