Currently, I am a Firefox user - but maybe not for much longer if they carry on like this.
First, they introduce Australis, and refuse to listen to any of their users complaining that it suffers from bad usability.
For a long time, I was using the full theme support, in order to not have to use crappy Australis. I stopped doing so, not because I don't want to use theme support, but because the themes themselves don't work with newer versions - continual bloody cat and mouse game.
I've never used tab groups, but maybe there is a reason for that - if you want users to use a feature, DON'T HIDE IT. Seriously, the average user would have no idea that tab groups even exist, because there is no button for it by default, no menu for it. You either have to customise the UI, or know an obscure hotkey.
I had switched back to Firefox because Chrome isn't as efficient as it appears to be. But at this rate, I'm either going to be back on Chrome, or going to Vivaldi. The only thing preventing me from giving Edge a serious go is a lack of plugins.