Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

• https://www.mozilla.org/en-US/firefox/all/

I am using Linux. Firefox hardware acceleration is automatically disabled - there is no support for it AT ALL. If you switch it on it does nothing.

I am using the default theme.

Using a manually downloaded version of Firefox sounds like a bad idea for ongoing security and stability. I’ll stick with the official repo thanks.

I will try Safe Mode eventually, but it makes using Firefox so difficult (e.g. not having access to a password manager) that it will be really painful running it long enough for this fault to appear.

I’m 99% sure this is related to the major changes that were released for Linux (Windows got them ages ago) with version 63.