If I recall correctly 73 has hardware acceleration enabled on Linux by default
for the first time. This will cause the problem you describe. Check the
about:config page and if you have:
layers.acceleration.force-enabled true
Thank you, but yes, absolutely I am using hardware acceleration. I have been using WebRender for a long time, last May, since before they turned it on by default in Firefox 68. While it certainly does not occur without WebRender (I did test that), it is not an acceptable workaround and not a solution. I'm not turning off hardware acceleration in my browser.
This is a recent problem, in the last few months, and does not happen in other window managers.