After upgrade from 73.0.1,1 to 74.0_5,1, firefox dumps core each time a webgl content is invoked.
Reinstalling 73.0.1,1 from saved pkg makes webgl content works again.
Iridium and Chrome works (so not an X11/OpenGL issue).
Same problem have been raised on Linux but have "mysteriously disapeared" (very helpfull).
I compiled the version from ports and crash happens as well.
Note: the message about a missing transform_feedback2" GL extension is displayed by working 73 version and by non working 74 so my guess is that it's not relevant.
Thanks for your attention. Let me know if I can help.