Mozilla is doing what Google did to Reader, just getting something power users use a LOT and canning it without considering the impacts.
They already have a diminishing user base, pissing off power users is not the way to solve it.
People don't use group tabs much, first because most don't open many tabs, second because they simply never advertised the feature that much, just a few weeks ago I showed this to a coworker who had no idea it existed, and he is a young EE quite savvy when it comes to internet stuff, so if someone like that don't know about, there is no way a common user would.
I only hope that, if they go on with this, a good extension will be provided, otherwise I'll have to go to pale moon or other, as currently I use both chromium and firefox opened at the same time in two different screens, and it is enough to not have this feature in chromium (never found a good extension), and I usually have well over 100 tabs scattered over half a dozen tab groups.
this is a big mistake, they want to make it easier to maintain, get rid of stuff like pocket or hello which nobody actually wants, and stop this race to the bottom with chrome, where you remove features and advanced access to the browser reasources and features.