function set_pageload(browser){browser.addEventListener("DOMContentLoaded", pageload, false)}
function add_tab(e){set_pageload(e.target.linkedBrowser)}
function pageload(){
    var doc = this.contentWindow.document

    // modify page using "doc" as "document"
}

window.addEventListener("load", function(){
    // Listen to first page/tab
    set_pageload(gBrowser.mPanelContainer.firstChild.firstChild)
    // Listen to other pages/tabs as they are opened
    gBrowser.tabContainer.addEventListener("TabOpen", add_tab, false)
}, false)

FFExt: Modify any page on pageload (including background tabs)

javascript, extension

JavaScript