Firefox gives me a black background when I try to start a WebGL application. That's why I think WebGL is bad. Some kind of virus, trojan or something like that.
WebGL is a basic feature of browsers these days, it's definitely 100% safe and not malware. I am also using the same version of Firefox (but the 64bit one), if it keeps not working for you perhaps your graphics drivers need an update, or WebGL is disabled in your browser for some other reason, in any case I'd go to this URL to make sure WebGL2 is working properly:
The black screen could also just be a loading issue, perhaps related to itch's recent outages, so hard-refreshing the game's page might also fix it.
I hope this helps.