- It could be useful to have the possibility of automatically grouping all of the instances of the same website in one single tab;
- Clicking on such a tab would open a dropdown menu in which entries are the individual instances of that website;
- The browser wouldn't group anything by default; user should toggle this behavior from one of the same-site tabs.
As for UX, two non-trivial cases come into my mind:
- User leaves the website from a grouped tab;
- User enters a website from outside an existing group.
The browser could move tabs into and outside the group each time the navigation bar loses user's focus (blur).