Solution found on this thread: https://support.mozilla.org/en-US/questions/1333571?page=1 Namely, by doing this: "Rolled back to 78.9.0esr and this solved the problem. Downloaded here: http://ftp.mozilla.org/pub/firefox/releases/78.9.0esr/mac/ This also requires turning off auto updates on Firefox via Preferences. The above webpage has many varieties of FF. So look for the US Eng version. I used this page: http://ftp.mozilla.org/pub/firefox/releases/78.9.0esr/mac/en-US/ Helpful