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This is due to DMABUF being temporarily disabled on Nvidia due to bug 1788573. Without DMABUF, Firefox has to introduce extra buffer copies to get the WebGL content rendered to the screen instead of simply sharing the buffer.
You can, however, force enable it temporarily until that bug is resolved by going to about:config
and enabling widget.dmabuf.force-enabled
. Or you can just wait for the Nvidia 545 driver, which should have this fixed.