I want workspaces, ideally in a sidebar like in opera. Arc also does workspaces well but Firefox doesn't have to go that far and have the tabs on the side too.
Ton of features and very nuanced customization (you can change pretty much all minutia of its UI, which is a huge plus over other stuff I used). Some noteworthy features I like:
Vertical tab bar with collapsible parent/child tabs
Tab Grouping, which seems to be functionally the same as workspaces
Unload or refresh tabs or groups en masse easily
Customizable new tab buttons. ie: you can have "new tab", "new private (or other container) tab", and "new lemmy.world tab" all at once.
Customizable movement of tabs to specific Tab Groups based on domain and/or container. ie: I have a "Media" tab group, and I set it up so any domain from youtube.com or dropout.tv automatically get moved from other tab groups over to "Media"
There's more, but those are the points that I find the most useful.