I find Librewolf to be a bit annoying as well. You can disable that cookie-deleting behavior, but I was annoyed that when I signed in with my Firefox account that it had deleted my history and synced that deletion, so now my history was gone everywhere.

Zen is my current favorite Firefox derivative. Whether you like it or not will depend on if you like their UI choices. I actually wasn’t a tabs-on-the-side guy, but after using it for a day or so I’ve come to like it a lot.

Waterfox was nice too, but it was a bit unstable last time I used it.