Sorry, but what's happening. Does the problem involve ? or A in square boxes instead of actual characters?

Those usually are associated either with (A) a particular font being corrupt, or (B) using a third party font manager which stores fonts in a location on disk that Firefox does not make accessible for websites. In those cases, a fallback typeface is used, which often just has symbols.

Does any of that sound like it might be the issue or is it something else?

Yes, the text appears as A in a square box. What I don't understand is, I'm using the same font management system we've been using for a long time and I've not introduced any new fonts. So, what is causing this?

Could you try the "sandbox" setting change mentioned in this thread to see whether it's that issue: https://support.mozilla.org/questions/1197246

Yes, that's seems to have solved the problem. Many thanks.