It’s really not bad. It is a change. You might consider keeping it enabled for a couple of weeks and see if you still hate it so much after the new wears off.
I barely noticed it even after reading HNers whine about it. They said it covered other UI components. In reality it grows a few pixels. I wouldn't have noticed it had I not read HN, that's for sure.
I still legitimately don't see what people are describing about the address bar. I haven't noticed a change the entire time I've used Firefox (about year). Could someone post a before-and-after screenshot or something? (I've been using Firefox Developer Edition too if that makes a difference?)