The game defaulting to fullscreen (or even rendering that way in fullscreen) is a bug either with Godot, with how I'm implementing the web export in Godot, or with a certain subset of web browsers. I've been trying to dig into it so I can avoid the issue in future- were you on Firefox, by any chance?
And yeah, the balance could use some work. At later waves, you start to see "supercooled" snow that takes a few hits and gets i-frames after being hit; the bluer it is, the more hits it can take, which means that eventually it'll get rough enough to just end the game. The issue is with how I implemented it, you don't start seeing that until waaaaaaaay too late in the game to really care (until it gets enough i-frames to just ignore all your defenses and end the game anyways.) I'll keep an eye out for that in the future.
Thanks for the feedback!