I guess it was the update to Firefox v. 86.0 here on Ubuntu that made Firefox create an extra empty window beside the session I always (re)open, which contains a large amount of tabs. When I try to close the empty window Firefox gives a warning, that this will close my (many) tabs. That doesn't make much sense to me. And when another app, like Thunderbird, opens an URL, it is opened in the new empty window. I can add that I use some add-ons like Simple Tab Groups, Vertical Tabs Reloaded, AdBlock Plus and NoScript.