Thanks for letting us know about this bug!
I've actually been having a lot of trouble getting the game to run on Firefox myself, whereas I have absolutely no issues on Chromium browsers. It seems the default URP Lit shader is causing the issue and isn't rendering.
I'm sorry for the inconvenience.
To get around this problem, I found that going into Firefox's 'about:config' and setting 'webgl.disable-angle' to true actually fixed the issue for me.
Since I can't upload a new build during the jam's voting period, it's the best I can offer right now.
I hope this little trick helps, or that you were able to try it out on another browser!
Thanks again for the report!