I assume you have all boxes checked? see screenshot

.....

Yes, most of the boxes are checked. Still nothing

I would try another account on your Mac, then open Firefox and sign into your account and see if sync works. The next thing I would try would be to go to your Firefox account and look at the connected devices, find the mac and sign out of it. Then go back on the mac and make sure the Firefox is not logged into and try again. Sync is showing my macbook from 10 days ago, but I was just on it last night.

I've never done it myself, so as a test let me sign out of my mac from my linux computer and see what happens when I log into the mac.

When I sign into my mac, I'm going to use another account just to see what sync does.

Cannot replicate the issue. I logged into my wife's account on the mac and then logged into my Firefox account and everything came over.