You should never manipulate native code like that. You should have code that does feature detection and does proper behavior.
If you manipulate browser prototype you run the risk of conflicting with plugins because no authors are expecting that. For example a chrome plugin overwrites a certain property which breaks TinyMCE. It's quite frustrating.
It's the web which is not a tight box so you can't pretend only you are using the browser.
This question shows you the right way already:
event.preventDefault() function not working in IE
This code from a comment is probably the best for you:
if (event.preventDefault) {
event.preventDefault();
} else {
event.returnValue = false;
}