I assume you mean "practically nobody without an adblocker". I dread the day we could have to live without Firefox. Don't dismiss it like that, please.
Mozilla has never been a competitive company. They've always leaned heavily on the illusion of being a non-profit. They've always sold the "eat your vegetables" version of web browsing, without actually offering anything compelling over the competition. We're just expected to use Firefox because "it's made by not-Google".
You were clearly the issue in the other thread.
With Chrome, Edge, Opera, and Brave all running on Chrome, we're getting awfuly close to a browser monoculture - one controlled by Google.
Supporting FF might be a pain in some cases, but failing to do so drives users away from FF in the long run, and drives away FF users from your site.
I know web devs have limited resources, but look at it this way: supporting FF and Chrome is a lot less of a pain than it used to be supporting IE.
My job is not to drive users to Firefox. My job is to create a product that people can use. And in my particular case, my job involves creating a product that doesn't physically harm people.
I agree that browser monoculture is a problem. But there is another problem that Mozilla is not the company that is helping with the problem.
EDIT: to go a little further on this, it's not physically possible to create a good WebXR experience in Firefox, at this time. Despite their announcements of focusing on Hubs as one of the few projects they kept around after the most recent purge, there are key technologies that are fundamentally missing from Firefox that have no replacement. And then they have the termerity to user-agent sniff Chromium browsers on desktop PCs and tell you that they supposedly don't support WebXR, that you need to download Firefox to use Hubs. So it seems even Mozilla doesn't really give a shit about the Open Web.
Why shouldn't the browser have VR APIs? WebXR allows us to iterate faster, across more devices, and more easily integrate with other services than building in a framework like Unity can do for us. I've spent the last 6 years doing nothing but WebXR and Unity work and WebXR is hands down far easier to develop the sort of application we're building.
No, you're not going to build MS Flight Simulator in WebXR, but you probably weren't going to do that in Unity, either, and ain't nobody got the budget for AAA++ graphics anyway. The folks who do graphically intensive work in Unity have stripped large parts of Unity away to make it possible.
I don't understand your last question.
Normally, the fact that practically nobody actually uses Firefox would mean it's not a huge problem. But the only standalone VR headsets with a Chromium-based browser are the Facebook headsets. If Firefox were better, I could be running twice as many of the not-Facebook-encumbered Pico Neo headsets for the same price. As it stands, Oculus for Business headsets are almost the same price as a Valve Index, once you add in the mandatory "support" yearly licensing fee.