European date format in Mozilla products

It actually is a system-wide setting:
      Locale's LC_TIME environmental variable
It sounds simple, but the value you have to choose is not so easy to guess.

# locale -a | less

should give you a hint.

Once you know your locale put it in /etc/profile to let it be a system wide setting. E.g.

export LC_TIME="it_IT.utf8"

Reload your X session and it's done