Type about:preferences#general<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1

Also see: https://support.mozilla.org/en-US/search/advanced?q=fonts&language=en-US&product=firefox&sortby_documents=relevance&a=1&w=1

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

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Some websites have enabled Brotli (br) encoding for files send via a secure connection. Some (security) software intercepts a secure connection and acts like a man-in-the-middle to scan content and may not recognize this encoding and changes the content-type header to text/plain. This causes Firefox to display compressed content as gibberish instead of rendering the decompressed content. You can check the issuer of the certificate to see what software is interfering with the connection and places itself as a "man-in-the-middle" between Firefox and the web server. Make sure to have the latest updates of this software and possibly contact them for support.

The Allow Web Sites To Choose Their Own option was already turned on, so I don't think that is the solution, but thanks for the reply

I tried safe mode, default theme, disabled hardware acceleration and none of these worked. Not sure what to do about userChrome.css. But thanks.