Session Isolation (Container)
In Firefox you have Container Tabs essentially superseeding Private Browsering mode. Container Tabs make it literally impossible for cross site tracking since every storage method is isolated ( test it at https://samy.pl/evercookie/ ). Even tabs in the same window (and same domain) can be isolated against each other.
Is there anything even remotely similair for Chrom* variants? Private Browsering is not an alternative since not only is it terribly annoying for regular use but also it can't protect you as long as you don't close the current window; if you are on https://vivaldi.net you have to close the entire window that has the https://vivaldi.net tabs to clear the storage making it unable to visit different toplevel domains. Chrom* doesn't even isolate multiple private windows against each other.
In Firefox you just open a new Tab in a new Container group. Regardless which window and they are completely isolated.
Tab grouping in Vivaldi would be able to become an option to allow multiple tabs with a shared isolated context without the overhead for separate windows or the need for container management.
Upvote the linked features as base technologies to get to this goal.