They have, it's suspected to be caused by a race condition with memory pressure events [0][1]. It looks like the patch is not included in Firefox 74 though.
I think it's tracked under https://bugzilla.mozilla.org/show_bug.cgi?id=1604492
It's still open, but it seems people haven't been able to hit the bug on the last few nightlies (this is also my experience), so it's possible the bug was quietly fixed as a side effect of some other patch.
[0] https://mozillagfx.wordpress.com/2020/03/02/moz-gfx-newslett...
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1617083