Install the beta version and you'll see it. My first impression was "ooh shiny I like it!" but the article has a whole lot of truth in it.
It's the same flat UI everyone roasted Windows 8/10 for going towards. Apple is doing it / has done it to iOS and OS X.
Mozilla shouldn't get a pass. They may do other things great, but UI isn't one of them.
The menu change in my opinion isn't too terrible or noticeable. The flat tabs are a pain though. I just deal with it in Chrome or Edge because they have other features that make them worthwhile.
All that said, I still use Firefox here and there mostly because of it's distinct interface. It's just an easy way to shift my brain that I'm working with X group of open windows/tabs vs what I got going on in Chrome or Edge.
I don't use it as my primary because of the features it doesn't have or support as well as it's competitors.
This would pretty much make me uninstall Firefox or use it even less since it will look just like Edge or Chrome. My only (and you can roast me for this since there's more elegant ways to do this) use case for it would be to test other sites and a hacky way to launch a browser on a specific monitor/resolution.