Hey, thanks for pointing out this issue.
I did a couple of play tests in Firefox just now and ran into the same issues you did a few times. After doing a bit of digging into it, my best guess is that this a WebGL on Firefox issue.
I followed the instructions in this link and I think it fixed the problem on my end. It seems Firefox has a setting called "webgl.force-enabled" which defaults to false, and this causes some edge case issues. After changing it to true, I have not run into the problem again.
I'll have a look at what is actually causing the bug on our end though, so that is not as inconvenient for future Firefox players.
Thanks for checking out the game!