tabgroup values: where initial use of Tree Tabs was preceded by use of Tab Groups, distinguish between groups

Example

With Tab Tree and About sessionstore after disabling Tab Groups:

There's a variety of tabgroup values – I have seventy or more groups across seven windows (in the shot above, four of eleven windows are group-free) – but if I close the about:sessionstore tab, no way to distinguish between groups.

Enhancement request

Distinguish between groups.

Maybe: for each tabgroup, take tab# 1 and make it the parent; with nesting at a single level for tab# values 2 and greater.