Problems using Google Classroom via Firefox and OS10.14.6

I am a tutor using Google Classroom, accessed via Firefox on my old iMac, running OS10.14.6. I could update the OS, but if I do, I think the old copy of Microsoft Word, which I keep for emergency use, will no longer work. Last weekend on Google Classroom I suddenly found I was unable to edit materials I had uploaded and, crucially, assignments that I had created, because they stopped appearing on the "material" or "assignment" page. The materials I upload are there, and usable by students, but, for example, I can't delete them. With the assignments, again, I can create them, but I can't select the option to send an individual copy to each student.

I'm not aware of any updates to my iMac over the weekend that would have caused the change, and I turn it off at night. I was told by the college where I teach that Google Classroom has not been updated recently.

I have emptied my cache and cleared cookies.

I find I can use Google Classroom as normal on my partner's Windows laptop.

Any ideas for fixing the problem on my iMac?