As a follow-up, I did a complete reinstall of Firefox with a new profile, removed as much cruft as I could (pocket, etc.), upped the content process limit... and I feel like Firefox is much more usable. There's still some lag, but it's more on the order of 0.05 - 0.1 seconds.
Now if I could just get it to be reasonable about search engines...
Running my mouse down the list in about:config feels like I'm drunk.
This is on a brand new maxed out i7 MacBook Pro.