Firefox Extension cannot override element events

I'm having trouble trying to override a form element's onsubmit event. I have no problem adding a listener with addEventListener, but for my particular case, I need to replace the onsubmit but for some reason when I do, it gives me this error:

Error: Component is not available = NS_ERROR_NOT_AVAILABLE

My code is simply this:

gBrowser.contentDocument.getElementById("theform").onsubmit = function() {
   return false;
};

Essentially I want to prevent the form from submitting, but this code fails and throws the above error. Using addEventListener to return false d开发者_StackOverflow社区oesn't seem to stop the form from submitting.

Thanks.

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For security reasons, the object returned by getElementById in an extension is an XPCNativeWrapper around the DOM element; it's not the element itself. This results in some important limitations. More details here:

Assigning to or reading an on* property on an XPCNativeWrapper of a DOM node or Window object will throw an exception. (Use addEventListener instead, and use "event.preventDefault();" in your handler if you used "return false;" before.)

• https://developer.mozilla.org/en/XPCNativeWrapper#Limitations_of_XPCNativeWrapper