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.

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?

After using it for a few days and having an account for a few hours (this is my first comment), I don't think it will ever directly compete. But I think it does have chance to represent a "significant minority" of internet traffic if it doesn't peter out early on, and it may already be passed the threshold for that happening.

You'd never say email can "compete" with twitter, but it's still a significant way people interact with the internet. If lemmy does for independent communities and niche forums what email does for messaging, I'd consider it a huge success!