After some updates this (known) problem is not fixed:
In summary, the graphical user interface of firefox only opens after one of several running processes has been terminated manually. All measures such as clearing the cache etc. and others as stated in the above thread do not lead to a consistent solution.
Whether this is a libx11 problem that can be fixed with an available AUR-libx11-firefox-fix is not for me and probably some others to judge (especially since it’s more of a startup problem than a freeze). If possible I would avoid installations from the AUR.
It would be very helpful and kind if you could take a further look.
Experiences with the AUR fix would also be helpful.