Sorry! The downloadable build might work better for you. It might also work without making it fullscreen (although that's obviously not as good, just as a test)
It seems to happen whether in fullscreen or not. This is the error that occurs:
Uncaught NS_ERROR_FAILURE:
createImageData https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
convertToData https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
copyPixels https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
copyPixels https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
addAtlasAssets https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
createAsset https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
getTilesetIndex https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...
Fa https://html-classic.itch.zone/html/9584115-911111/enter-the-titan/EnterTheTitan...