Pluto

Vlad while I actually like the overloaded Safari 15 compact tab bar, I get it's controversial and indeed not perfect — irrational I know! "Hey, a new shiny to play with!"

For me the slim approach you've shown here is the most user-friendly as it reserves maximum toolbar space to you know, actually grab and move the damn window! That's what IS sorely lacking in both the Safari 15 compact tabs and in Firefox's approach.

So, as much as the new Safari tabs are "fun" to be pragmatic I'd vote (fwiw!) for you to stick with your current approach and 'just' optimise via your "Compact Size" approach to balance space and keep maximum usability.

Whereas the IE-like "compact tabs with fixed position address field" feels like it could be a bit unnerving / confusing for a selected tab to 'jump out' of its previous tab order — when that tab order (by default) can give context as to when/where/how a new tab was opened, but only if default behaviour were to open new tabs next to their parent, instead of your current approach of adding them to the far end of the 'tab stack'.

For reference this is what trimmed compact mode looked like (we were much more compact that Firefox compact mode)