The second reason why yyyy-MM-dd is the correct way to write days (beside being sortable by default) is because even Americans doesn’t have any similar format that it can be confused with.
I learned this in my first half year as a programmer I think, somewhere well over a decade back and so far it seems that guy was right.
I expect you have tried to search for it in Firefox, but have you tried sideloading it somehow?
As far as I know Firefox supports a superset of the APIs that Chrome support, so it should be a possible?