Somehow they managed to have a huge market share when Microsoft was fighting tooth and nail in courts and out to tell people to use IE, and Microsoft WAS home computers.
Websites arent made with firefox, so they are not optimized for firefox.
They also can be buggy, firefox is my daily browser, yet, multiple times per year, a website is simply bugged on firefox.
Google Chrome was recommended by power user, and that how they took the market lead, not because they are the internet, but because power users told regular users to use chrome.
Folks mistakenly associate "breaking" with "not being completely compatible with an illegal streaming website with improper TLS/cyphers." That's just an example but you know what I mean.
Those are the types of websites that have issues with firefox, because mozilla maintains it's own root trust store held to various standards/compliances, and many..less savory web addresses use not HTTPS content on their HTTPS pages.
This is a feature/value added bonus, but because it "breaks" sites for people....back to chrome.
Most are due to the firefox tracking protection.
Others (with everything disabled) were for an airline company, some french governements website, and recently an SVG editor that just call to use chrome and doesn't works on firefox.
Killing the servo project.
Running metaverse projects instead.
Yes, mozilla could do better.