how to run greasemonkey script before the page content is displayed?

Question

I am writing a plug-in for Firefox and using greasemonkey script to do that (I compile the user script using this tool http://arantius.com/misc/greasemonkey/script-compiler)

Answer 1

Using the @run-at document-start as suggested in the answer by @kwah my script executed before the content I wanted to manipulate was in the document body. As a work-around I set up an interval running my script 5 times/second until document.readyState == "complete" (or until 20 seconds have passed).

This worked well for my case:

// ==UserScript==
// ...
// @run-at      document-start
// ==/UserScript==

var greasemonkeyInterval = setInterval(greasemonkey, 200);
var greasemonkeyStart = (new Date()).getTime();

function greasemonkey() {

    // ...

    // waiting for readyState (or timeout)
    if (
        (new Date()).getTime() - greasemonkeyStart > 20000
        || document.readyState == "complete"
    ) {
        clearInterval(greasemonkeyInterval);
    }

};

If you're more sure about that the content will be there at documentready I think something like this would be more preferable:

function greasemonkey() {

    if (
        document.readyState != "interactive"
        && document.readyState != "complete"
    ) {
        return;
    }

    // ...

    clearInterval(greasemonkeyInterval);

}