I think it was just stubbornness, pure and simple.
Firefox actually had a feature like tab groups a long time ago. It was removed for "low usage" and ever since then they have been resisting reimplementing it.
Anyways it still needs improvement but I am very happy to see this finally land. At work we have been moving everyone off of chrome after the manifest v3 shenanigans and the lack of tabs groups was a long standing sticking point for some users.