I had similar problems when I first started using firefox.
It appears that IE is more forgiving when it comes to code and especially url addresses.
In order for firefox to display your page properly you have to have the proper extension at the end of your url.
While https://mysite/index will work in IE
Firefox would require https://mysite/index.html
I assume the same would be true for the php extension.
I’m new to wordpress and php but I’m almost positive this should help you.
