We built the engine and tooling ourselves during the jam in Typescript. We used ajeeb-ecs as a starting point though, since writing a performant ECS library takes a while.
Not much reason for starting from scratch other than enjoying the challenge of it, though I ended up liking the idea of the game enough that I wish I hadn't!
If you're curious about the nitty-gritty, the bug was due to FF's partial support of createImageBitmap() which creates a GPU-owned texture that can be (relatively) quickly blitted onto canvas (which we use for rendering from our tileset). Apparently one particular overload of the function -- which is the first example on the Mozilla Developer Network's documentation for the function -- is supported by Chrome but not Firefox, and that isn't mentioned anywhere on the page except the compatibility table. Live and learn!