And for a majority of those, that was intentional (aka no actual issue, switchtig the user-agent “fixed” the site).

same here and it became bad enough to force me to install an extension to switch the user agent dynamically.

So I have little understanding when people say they can’t use Firefox because pages break.

goes to prove the depths of the lack of technical illiteracy that the western world has been shepherded into.

Just my personal opinions (ranting) without hard data/facts: Which is basically back to the same issue from the IE days. Chromium is better than IE really because it can be used to make other browsers. But is being coded for by sites like IE in how if the agent seen is not Chromium then it hard stops. Which to me just makes the circular feedback that Chromium IS the standard because more people use it, but people use it because non-Chromium browsers "can't" load the pages they go to. Same applies to extensions being gutted because Google controls basically what gets to still work, and most Chromium browsers don't host their own extensions stores. Given most people don't bother trying other browsers because Edge is the default in Windows (so Chromium) or because a long time ago they were told "don't use IE and use Chrome" by their "tech friends" when IE was getting worse and worse with malware exploits. Younger folks have grown up with Chrome being "the internet" like IE was for folks in the 90s/00s.

A lot of the folks that do intentionally install Chromium "forks" do it for some misc features/layouts while being told sites that work in Chrome will also work. So those folks at least are interested in trying stuff like Brave/Opera/OperaGX after a YouTuber or some source they trust. But would only try FF or browsers based on it if those same trusted sources went hard pushing it, and if major sites they use (that currently artificially don't work) did work. The moment one site they use enough doesn't work, then they basically swear off FF and maybe even continue the telephone game of telling their friends/family FF is bad/won't work. Then the sites continue to code for Chromium because "no one uses FF (or whatever non-Chromium browsers that aren't Safari).

To be fair, changing your browser agent is on the technical side of things, in my opinion this goes beyond "refilling washer fluid in your car" but more like swapping out the licence plate light for a different colour - few people realise that's even a thing you can do, and that it has any effect.

Moreover, users should not be blamed when it's clearly the other party who's being willfully obstructionist. But I understand your sentiment. It's tiresome, right?