On Wed, 20 Nov 2019, Dr Rainer Woitok wrote:
Well, in my case it's just another page in the same desktop. I'm not sure whether or not that matters. But above of that it's perhaps time to compare Fvwm and perhaps also Firefox versions.
What versions do you use on what distribution?
On OpenSuse Leap 42.3, fvwm 2.6.6 Mozilla Firefox 60.0.2 (as I said, I do not routinely use FF, I vastly prefer Pale Moon !)