Nobody said a website didn't work on Firefox. Tough Microcock Teams doesn't work, I didn't find any other sites not supporting Firefox.

The market share is so low because of the same reason Linux's share is low: people use what most people use. When they get a new computer, they either don't know much and stick to Edge (which is Chromium) or install Chrome because that's what they are familiar with, and the reason they're familiar with it is because most people used that, so they also tried that. If they use other browsers, they just don't care enough to switch, no matter if it's much better or how easy switching is.

Pre-installs are also a reason, as I've said before about Edge. So if a well-known computer manufacturer put Linux on most of their laptops and a new computer user would buy one of them, they would just use Firefox cuz that's what pre-installed on most distros, and if more new users buy it who don't know about Chrome, Firefox market share becomes even bigger.

Most people just don't care enough to switch if their current setup works. Let it be Linux, Mac, Firefox or any less-used product.

Mobile browsing altered the landscape.

Early chrome introduced a lot of features that attracted the tech crowd in the late 00s and early 10s like html5 video (replacing the need for flash plugins) as well as multithreaded tasks for better use of those newfangled multicore cpus people were buying.

Around the same time firefox was experiencing a memory leak and took too long to open(I switched over to linux at the time and didnt have the issue but it was a big complaint online).

Early on firefox was also slower to add a lot of the newer html5 stuff that was popping up around the web while chrome was more or less built from the ground up with it in mind which also lost it some enthusiast mindshare.

Along the same time google started a heavy chrome push. Of course the default browser on android was based on chrome, and google search results pushed people to download chrome, and youtube's html5 video ran better on chrome(firefox initially didnt have all the codecs they used due to licensing). Google used its web dominance to advertise, and push, and advertise, and push.

Eventually firefox got to a point where they mostly caught up but by this point chrome had gained a solid footing and lead and here is where dirtier. Firefox was constantly behind on webstandard and synthetic benchmarks but many of the things firefox was behind in on these benchmarks were things that google had just introduced. Also with the new google dominance came lazy developers who would instead of building a site for web standards and test on multiple devices, build their site to run on chrome and bugs on other webrenderers be damned. With other major competitors like opera and Edge switching to blink it meant that the devs were mostly covered.

So this gave the appearance that firefox was still technically behind even after having closed the gap. And by the time chrome started having its own PR issues and memory leak problems the fluid tech landscape has changed.

Firefox lost market share in an era when tech and software were still fluid. New social media could rise up any day myspace could get killed by newcomer facebook. There can be multiple video sites who will win? IOS, Blackberry, Palm, Winmo, Nokia, and Android were going head to head in the smartphone space. Internet explorer, firefox, netscape, opera, chrome and safari all had different engines.

Nothing came to knock facebook out and the social media that rose up after was just different, youtube is essentially a monopoly in that space, IOS and Android are all that's left of the smartphone war, and firefox still exists but chromium's engine Blink powers most alternative browsers and firefox's usershare is tiny now. Safari only has a marketshare because it's default on macs and the only game in town on IOS. Chrome will have to really really really mess up in order to actually shed significant mainstream user marketshare.