I keep getting Out Of Memory errors using Excel 365 in Firefox 79+. This is obviously nonsense, since there are no web APIs to detect an out of memory condition in a web browser, and regardless I get this no matter how much RAM I have available (currently ~9GB). It only happens in Firefox, not Chrome. I get this no matter what document I open, or even in a new document. This first started happening sometime between 22 June and 6 July.
What is really interesting is, sometimes I can just hit escape a few times and Excel actually works just fine. Other times it says nonsense like "We're opening this in Microsoft Excel," and then "We opened this file in Microsoft Excel on your computer." which is particularly amusing because not only do I not have Excel installed, it is not even available for my operating system! (Typical lazy Microsoft programmer arrogance.)
Also of note, I use basic safety precautions like Tracking Protection, first party isolation, blocking 3rd-party cookies, etc. which I am absolutely not going to budge on. If Microsoft can't figure out how to write a web app that functions with these basic privacy protections in place, they really need to start hiring competent developers. I don't have time to debug this for them. I don't have any of these problems with GSuite Google Apps, which I've been happily using for years. Trying to use this Microsoft garbage really feels like going back in time.