Thanks for the report! Firefox was the browser I used during development and also needed the most work regarding performance. The image quality is heavily tied to rendering performance, so I’ll need to consider ways to improve this further.
Another tip is to make scrolling or some keyboard buttons rotate the pieces, snapping to 90° angles would be a good start. Making the pieces only snap when placed down instead of just when they are close to a compatible piece might also be good.
These things could be togglable or vary between the difficulty modes.
In any case, cool project and good luck with it!