This was true during an intermediate period where all browsers were more-or-less equivalent. It wasn't true before then, when many sites were designed with Internet Explorer in mind, and tended to work less well on other browsers. I don't think it will be true now, either, now that most sites are designed with Chrome in mind, and tend to work less well on other browsers.
The big difference is that the functionality problems 20 years ago were easier to explain, and therefore easier to get people upset about. It's a lot easier to weave a compelling political story about straight-up incompatibility than it is to weave one about degraded performance due to differing just-in-time compiler optimization behavior.
Also, we seem to be stuck in a situation where people are still so fixated on a monarch that hasn't been in power for over a decade that they maybe haven't been so concerned that the old monarch's overthrower has consolidated power to become a new monarch.
There seems to be this idea that some sites work worse with FF but in all the time Chrome has existed, I've never found any sites that didn't work with FF.
I've never found any that don't work, but Firefox supports fewer "standards", so you can end up with a somewhat degraded experience on some sites. My own company's product, for example, has some janky bits on browsers other than Chrome. The performance is only a little bit behind on most benchmarks, but isn't anywhere near as fast for certain kinds of animation. It can be noticeable on sites that run a lot of animation. Like, say, sites with ads. Possibly only if you're using an older or less powerful computer. Which isn't how we of the orange header bar like to roll, but also isn't a terribly uncommon thing to do out there in the wider world.
(Scare quotes around "standards" because calling Chrome-only things standard nowadays seems a bit like calling AcitveX a standard 20 years ago.)
I use Chrome almost exclusively at work, Firefox on my gaming computer, and Safari on my personal MacBook. Other than extensions on Safari and minor differences in keyboard shortcuts, I don't really notice the differences.
Maybe it's ublock, but I've had plenty of sites that just don't work on firefox. barclaycard, blackboard, and the schools web print system (dts) are my recent examples.
The big difference is that the functionality problems 20 years ago were easier to explain, and therefore easier to get people upset about. It's a lot easier to weave a compelling political story about straight-up incompatibility than it is to weave one about degraded performance due to differing just-in-time compiler optimization behavior.
Also, we seem to be stuck in a situation where people are still so fixated on a monarch that hasn't been in power for over a decade that they maybe haven't been so concerned that the old monarch's overthrower has consolidated power to become a new monarch.