On my iMac, Thunderbird starts, I get the full menu across the top for about 10 seconds then goes to one menu item in the top righthand corner which is "Thunderbird" and there is nothing displayed i.e. the screen just shows my desk top. The Icon in the Dock shows the Thunderbird icon with the number of unread messages in a little red circle by it. I have no email folders, nothing other than the one menu item. If I click this it gives me my preferences, services etc.

I am currently on Yosemitie 10.10.5 which I have been using for quite sometime. To try and fix this I upgrade to Thunderbird 45, it upgraded OK but still does not display anything once it has started up.