A few months ago I switched to Firefox in place of Safari under Mojave (which no longer gets security updates). Today, after updating Firefox I found that some of the old-style webpages that I maintain no longer displayed the letters Y and Z.
It turns out to be an old problem with Firefox using website-defined fonts rather than defaults. I followed advice here and the pages now display correctly:
I will look at editing my web pages to use a more reliable font. Until I do this you can “test” your Firefox settings here: