I've tried to switch from Vivaldi to Floorp and there is some things that Firefox does that drive me absolutely nuts.

The main one is the behaviour of pinned tabs. Pinning in Firefox turns it into an icon that is harder to hit and doesn't even protect it from closing. This makes them essentially useless, they should be moved to the front of the tab bar and be protected from closing.

The second is that when you use vertical tabs the tab bar acts like a title bar instead of a separate entity. This means you can't double click to create a new tab, and trying to drag a tab often results in the entire window moving. I have to use Tree style tabs and disable the normal tab bar completely to prevent this.

There are also things that I don't like such as how downloads are handled and I've has issues with my session tabs being saved properly.