Try changing your default font in settings. Click the 3 lines in the top right corner of your browser then scroll down to fonts and colors. Choose the font you want in the fonts and colors drop down menu. Hope this helps.

To help pinpoint the problem, you could try starting Firefox in troubleshoot mode (see link below). Possible candidates are 1. you have set the following preference in about:config to true: privacy.resistFingerprinting 2. you need to disable or change the hardware acceleration setting. On the General page of Options, start by unticking/unchecking "Use recommended performance settings". https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

Thanks Aaron and TNorth Did disable hardware acceleration, rebooted and problem is not seen.