I have the same issue on all my installs (unstable and testing branches) and for the sake of me i could only relate to this
and was also mentioned in
But it should have been fixed by then with the latest updates. The thing is, i see it on unstable too.
Edit:
It seems that, alt least on Gnome, when i run Wayland session instead of X11, that coredump does not take place. Will try to see if is the case on KDE Plasma, even tho for me was quite unpredictable and got random results every time.
Edit2:
On KDE Plasma and Gnome, both when using the X11 session the coredump takes place; when using the Wayland session, on both that coredump is not happening.
Stil no exact reason why is happening, but will investigate more.
Edit3:
Reading trough the Firefox - ArchWiki i found out that by removing the libva-vdpau-driver package from my install, rebooting the system and starting Firefox does not produce the crash anymore. I still get the:
$ firefox
[GFX1-]: glxtest: VA-API test failed: failed to initialise VAAPI connection.
when running it from terminal, but there is no more any coredump. Not observing any other issue, but will investigate more.