I was hoping that Waterfox would somehow skip this change, but it just updated a few days ago. Now that the new UI is out of beta, I don’t see a way of turning it off.

For me, the most irritating part is the way that it jumps a little bit if you only scroll a small amount. Small enough to make the Private/Tabs/Sync bar show up, but then immediately hide itself again. That happens in both grid and list mode.