This is absolutely THE feature that has kept me away from FF for a long, long time. I like most other stuff about it, but tab groups are a necessity for me. I exclusively use browsers with Tab Groups and Tab Stacking
Maybe I never learned how to properly use grouping in Vivaldi, but I concluded that Simple tab groups extension is better for my use case. In Vivaldi it seemed too simple to accidentally open new tab in another group. With STG extension I am always locked in 1 group and do not even see tabs from other groups.