When I've had my browser open for a while and I am using multiple tab groups, closing the selected tab sometimes causes the selected tab group to change unexpectedly. The tab I close when I see this behaviour is not the last tab in the group. To reproduce create multiple tab groups, and open sites between each of them in turn (some in tab group one, some in tab group two, some more in tab group one, some more in tab group two etc) then randomly select a tab and then close the selected tab via the tab strip and eventually the tabs panel will change.
I experience something close to what you've described. I suppose tabs have got a birth date, or an ordinal number if you prefer (first, second, third, ...etc). If we have a single group, with several tabs. closing one of them randomly, will display the next (obviously on the right side) If we have two or more group, with tabs inside. I add a new tab (global number is n), tab[n] in group[a], then i switch to group[b] an add a tab[n+1]. I get back to group[a] and delete tab[n]. on ff41 + profile 1, I'm switched to tab[n+1] in group[b] on ff41 + profile 2. I stay in group[a], on a older tab (right side of former tab[n]) I don't understand... following more information about my profiles Profile 2 is clean, only two groups and a few tabs to test, Tabs were added through the cross Profile 1 is running for several days nows, with 4 groups and more than 30 tabs in history, some of them were opened from links in others. Will come back later. I still wonder how are numbered tabs