I made a User Script. If I append the script to the end of a webpage, it works fine. If I load it into Greasemonkey, the script doesn't run when it should. It's not a problem with the include filters, because Greasemonkey says that the script is activated when I'm on the page, and if I replace the script with a simple document.write command, it executes fine.

This is the script:

var value = document.forms['thisform'].selectedOption.options[0].value;
document.forms['thisform'].selectedOption.options[0] = new Option('New Option',value);
document.forms['thisform'].selectedOption.options[0].selected = true;

var value = document.forms.namedItem("thisform").selectedOption.options[0].value;
document.forms.namedItem("thisform").selectedOption.options[0] = new Option('New Option',value);
document.forms.namedItem("thisform").selectedOption.options[0].selected = true;

It's an XPCNativeWrapper's limitation