Hi John

I have been using Gmail in Firefox for quite a few years without any issues. Do you have any add-ons installed in Firefox?

1Password, Duck Duck Go, Pinterest. That's about it. If I clear the cache and then reboot my computer (MacOS) it's reasonable for a day or two, then slows down again.

When clearing the cache works well -- especially without changing anything else -- it suggests that disk access is slower than your network connection, so caching does not optimize overall performance. I don't know whether this is due to a shortcoming in Firefox's cache design, or perhaps your disk is very fragmented. But if that resolves the issue for a few days, I would suspect disk fragmentation, or some kind of cache inefficiency.

This is a MacOS drive formatted as APFS, so it defrags automatically and they specifically say don't run third party defrag on it.