I had to run this game in Edge, it really doesn't like Firefox. The mouse just points to the sky if you move it in any direction.
Egg.
This actually caught me out just as I was uploading! Did almost all the development for the game in Firefox on Linux, but seems like there's a specific issue with itch and godot mouse capture. I added a comment to an issue I found on GitHub about it - if there are any developments I'll update: https://github.com/godotengine/godot/issues/51810
(In the meantime, it worked for me when I played in fullscreen, at least)