Question

I need a way to load a website – something like gBrowser.loadURI, window.location or window.open – but I need to execute some more code AFTER that website has been loaded (and parsed by the browser). The functions I’ve mentioned don’t block execution of my code until the site is fully loaded, but only until it has started loading.

In case it matters: This code will not be part of my/a website, but will be a FireGestures script.

Answer 1

https://developer.mozilla.org/en/Code_snippets/Tabbed_browser#Manipulating_content_of_a_new_tab seems to be what you want. They suggest:

var newTabBrowser = gBrowser.getBrowserForTab(gBrowser.addTab("http://www.google.com/"));  
newTabBrowser.addEventListener("load", function () {  
    // use newTabBrowser.contentDocument to manipulate DOM
    // or do whatever you want on-load
}, true);

See also docs for tabbrowser and browser.