Can anyone shed some light on why this is producing an error in FF.

Code:
<input type="button" name="my_btn" value="Click Here" onclick="for(var j in this) { alert(j + ' : ' + this[j]); }" />
Here's the error
Error: uncaught exception: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMNSHTMLInputElement.selectionStart]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: http://localhost/error.html :: onclick :: line 1" data: no]