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.