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.
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.