I think I found the culprit. What seems to be keeping Firefox from preventing sleep is a tab I have open to Discord. I have my Discord tab pinned so it always loads. If I go into about:processes and unload that tab then Firefox stops preventing sleep.

So now that I know it is not the latest version of Firefox, but rather a bad behaving tab, what can I do about it?

I solved the problem. I have the default Autoplay permission set "Block Audio and Video". But at some point I gave the Discord tab an override permission to be able to auto play audio and video. That tab was what was keeping Firefox from disallowing my Mac from sleeping. I am not sure how long this has been going on but I became aware of the Mac not sleeping around the time of the last Firefox update which is why I initially thought the two things were correlated.