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 or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• 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/#product-desktop-release

In another thread, someone mentioned this as a possible side effect of the FB Purity extension.

Both replies helped me find the problem. I'm pretty sure it's a combination of YesScript2 AND FBPurity but at any rate, it's not Firefox that Facebook hates, just things that interfere with its chosen JavaScript functions.