I can't ever go back after moving to Vivaldi, even though it's chromium. It's the tiniest of reasons too, like full screen behavior, the gargantuan UI elements, the continuous removal of customization options...
I can set Vivaldi to:
F1 = show/hide info bar
F2 = show/hide address bar
F3 = show/hide tab bar
F4 = instant fullscreen
F5 = sidebar that's actually useful
The bookmark and history tabs are actually usable, the Quick Command popup bar functions like Vimium, and there's more customization options than you can shake a stick at.
I say this as a Firefox user who has not changed browsers since it was literally called "Mozilla" and competed with "Mosaic" and "Netscape Navigator" for top position as THE web browser to surf Internet portals.
I love FF to pieces, but it's simply not usable to me under the current management.
I just tried out Floorp which is basically Firefox with some Vivaldi features. I was like oh okay I get the appeal now. I have to take Vivaldi for a test drive.