I think you're being a little excessively hyperbolic. They don't support your browser yet and state as much - that's hardly malicious or demeaning to you. They also promise support is coming soon. It's a very early phase product that uses a lot of newer and less universal APIs. It's not that they have so many users that they don't care - rather, it's that they're still beta testing and haven't been able to devote the resources yet. I really fail to see the problem here.
I have a similar antipathy for Quora for requiring facebook logins to view their content (not actually theirs, their users' content) that came up during search results.
I believe they've now dropped the facebook login requirement, and I think they now show a lot of content from organic search traffic, but I plan on ignoring them anyway for as long as possible.
And it's not just non-proprietary browsers, it's just any browser not cherry picked by the developer, which for some inexplicable reason includes the iPhone but not Firefox, Edge or Opera.
edit: I didn't actually try to sign up for early access.. it's completely possible that that portion of the site doesn't work in Firefox, but there's no reason to block users from even seeing what it's about...
If this is true (I don't know), we should blame the lack of standardization instead.
I love more than anything else, sites that treat me as a second-class citizen for using a non-proprietary browser. They must have so many users already, that they don't mind alienating a large portion of their audience.