So, I did some investigation and know what's broken, but I don't know why. The game uses `canvas.toDataURL()` to replace the favicon for each frame. It draws it, essentially like this jsfiddle https://jsfiddle.net/trLkd1ys/

The fiddle works in both Chromium and Firefox for me (a 16x16 green filled square), but the data URL produced by Firefox results in vertical line rainbow. In Chromium it works fine.

FF: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAQUlEQVQ4T2O83OR55grL9sMTZ1tdefydOU2t5o50tbH8dc+a8h8pYZ8fZ71cboZPnnHUgMejYfB5NAyAeWUYpAMAFY8jzzH0m5oAAAAASUVORK5CYII="

Chromium: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAAXNSR0IArs4c6QAAAB5JREFUOE9jZGhg+M9AAWAcNYBhNAwYRsOAYViEAQCDTRgBEA8acQAAAABJRU5ErkJggg=="

https://onlineimagetools.com/base64-decode-png can be used to quickly see if you're inclined.

works fine on firefox 108 here on fedora

It displayed a random rainbow of vertical lines for each frame. Not sure why, and didn't attempt to debug.

Works for me, FF 112.0.2 on Arch.

Hmm. Same version for me. The only difference is my FF comes from a Snap. Perhaps that has something to do with it.