I just had the same issue on an application that is loading a script with a relative path.
It appeared the script was simply blocked by Adblock Plus.
Try to disable your ad/script blocker (Adblock, uBlock Origin, Privacy Badger…) or relocate the script such that it does not match your ad blocker's rules.
If you don't have such a plugin installed, try to reproduce the issue while running Firefox in safe mode.