I needed to disable the Enhanced Tracking Protection button (under the shield by the https lock icon in the URL bar) to get it to work, but otherwise firefox works fine for me on Linux with this site.

I dont know what to tell you either.

I hate "It works for me" as the end of troubleshooting.

But whether you can replicate the issue on other systems is valid diagnostic information.

It could legitimately still be a firefox bug, one that relates to your system in particular. But I have no way to tell from over here.

most likely a browser plugin then, assuming you've checked the console for errors?

The only plugins I use (as you can see in that screenshot) are Bitwarden and uBlock Origin. Disabling uBlock had no effect. I'd be shocked it Bitwarden were causing it.

I checked the console for errors. There were two warning about deprecated API usage and nothing else.