This isn't a direct replacement for tab groups, but there's a Firefox extension called Tree Style Tab that organizes your tabs into a nested tree structure. I use it a lot to emulate tab groups and the way it lays out the tabs makes it much easier to read imo. It might be worth taking a look if tab groups are chromium's "killer feature" for you.
If you don't mind me asking, are there any other must-have features that chromium has that Firefox doesn't?
I actually really like Firefox for reading pdf's, how is it in chrome? I've never actually tried chrome for that because I was still using okular back when I still had chrome installed on anything.