How unfortunate, can you try opening the browser dev tools (ctrl+shift+i) and go to the console to see what messages are in there?
Right, it seems to be because it's attempting to access the browser storage, and isn't allowed to by the browser. (There appears to be some max size limit imposed by your installation of Firefox, perhaps due to incognito mode) There is some code in the game that should protect against this, but it appears it isn't working in a 100% of the cases. Thank you for reporting! I will put it on the list with things that need to be fixed (for now, using another browser or going out of incognito mode should work)