hi jonbarril, that's likely an issue with hardware acceleration - please try updating your graphics driver, or in case this doesn't solve the issue or there is no new version available at the moment, disable hardware acceleration in the firefox menu ≡ > options > advanced > general (that setting will take a restart of the browser to take effect).

I have the latest drivers. I checked with the laptop manufacturer and they said the problem was unique to Firefox. I have only seen the problem with Firefox -- not Thunderbird, or any other product/app. What could Firefox be doing that no other app is doing with regards to page rendering?

Also, I tried disabling hardware acceleration, as you suggested, but the problem persists.

have you restarted after this config change?

Just tried again with a restart. That fixed it. (Of course, using Chrome also fixes it;-)