I don't really use firefox or have any experience with GDevelop (Which I think the game was made in), but I had a quick look at what source was available to view from using Brave's developer tools and the javascript section seems to reference Mozilla's extensions relating to WebGL, though when I looked them up; it seems some references are depreciated, so maybe GDevelop hasn't updated their code base for exporting to HTML5? If so, I dunno if the developer can do much about that.
Though a lot of the time, even when an extension is depreciated; browsers will still allow for it (from my understanding). I'd recommend opening the developer tools after the game loads and checking if there is a recurring error in the Developer console (if you're interested in that kind of thing).
I think a lot of web developers mainly develop using a chromium based browser in mind, then patch compatibility for firefox and other browsers, so a chromium based browser is probably just better for web games right now.