In firefox 3 in Rawhide, when you select "Print to File" as the printer and specify the name of a file that already exists, it happily overwrites that file without prompting for confirmation. Mozilla used to ask for confirmation, as does Seamonkey; I can't remember whether older versions of Firefox asked for confirmation.
We are fixing this upstream: http://bugzilla.gnome.org/show_bug.cgi?id=474302