Your tilemap has over 16,000 tiles in the tileset image. Is this something you really need? It seems like a case deliberately made to be slow. C2 has to cut up the tiles from the tileset image when the layout starts, and it seems Firefox is slightly slower at doing this, and takes long enough to show a warning.
Unless you have a real-world tilemap that does this, and you can't make it smaller, closing as won't fix.