This suggestion is for a configuration addition that would allow for new tabs opened from an existing tab to always open at the end of the tab list, even when the existing tab is in a tab group.
Current behavior is based on the "browser.tabs.insertRelatedAfterCurrent" about:config option - when "true" (default), a new tab opens immediately after the current tab, when "false", a new tab either opens at the end of the tab list (if current tab is not in a tab group) or at the end of the group (if current tab is in a tab group). This would be a new configuration, possibly "browser.tabs.insertRelatedInGroup" ("IRIG"), that would default to "true" and would work in tandem with "insertRelatedAfterCurrent" (IRAC) in the following fashion:
IRAC true, IRIG true: new tab from grouped tab opens immediately after current tab, within group (current behavior)
IRAC false, IRIG true: new tab from grouped tab opens at end of current group (current behavior)
IRAC true, IRIG false: new tab from grouped tab opens immediately after current group, ungrouped
IRAC false, IRIG false: new tab from grouped tab opens at the end of all tabs, ungrouped