Ctrl-click on the Internet Explorer application, choose "Show Package Contents". Open the Contents folder and then edit the Info-macos.plist in your editor.
Close to the end you see*
(* the space in < key> < /array> etc, is only to show them in code.)
LS is OS X's Launch Services which handles all the file typing and associations for the OS. Deleting these lines should prevent IE from hijacking the browser.
Also, renaming the file /System -> Library -> PreferencePanes -> Internet.prefPane -> Contents -> Resources -> English.lproj* -> DefaultHelperApps.plist to something else, then quitting and relaunching the preferences (and log out/in) could do it. (*Or Italian.lproj, whatever you use your system in).