Dunno, it's not working for me either. Your best bet is to ask the game's creator. Comments are open.

This sus game works just fine for me, but I have problems with firefox too. In my case when my 3d game is capturing my mouse it moving my camera to the right on it's own with the slow speed. And the strange part is it works just fine in Edge and in firefox when I open it on full screen. Also I tried to run my web buid on simple local python server and went on localhost on firefox - the game works just fine.

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.