They're pretty much just hating to hate or basing themselves on very outdated information, 'missing critical features' is a joke, because if it actually were critical it would've been implemented already (plus firefox is very extensible, with many plugins existing and forks adding specific features), if they actually had a point they maybe would've given a single example.

Weirdly implementing some web standards kinda did apply a bit until a few years ago where all the big browser engine developers got together and pinned down the standard. If something still breaks that probably means the website used some out-of-spec workaround that only works in Chrome. Some things do indeed behave differently between firefox and chrome (an example of my own: file input fields with multiple types, eg allow both video and image are handled differently at least in the mobile apps). Yet again if they had a point maybe an example would've been great.

Weird user agent styles?...?? I'm just confused honestly.

Me too