When I close a tab in a tab group (e.g. A) it sometimes happen, that FF switches me to another tab group, let say B. This phenomenon is reproducable. I think the problem / trigger is, that FF has an internal numbering of the order in which tabs across all tab groups were opened: Example: 2 tab groups A & B and several tabs opended afterwards in different tab groups A: 1,2,3,4, 8,9, B: 5,6,7,10 When I now close the forth tab in group B, opended as the last one (10), FF will automatically focus on the last tab in group A (9) which was opended just before the 10th. This happens a lot of time! And I can reproduce the problem. I think these post describe somewhat but not exactly the same problem: https://support.mozilla.com/de/questions/828677 https://support.mozilla.com/de/questions/837274