Question

How can bind onload event on “body” element of second tab in Firefox extension?
I can’t use window.onload ’cause it binds on current tab.
So how can I do that?

Answer 1

Ok, I didn’t read the part about opening a new tab.
We’re doing it with this code in the SendToPhone extension:

var tab = gBrowser.addTab(url);
var c2pTab = gBrowser.getBrowserForTab(tab);
//Add listener 
c2pTab.addEventListener("load", function () {
    // do your stuf
}, true);

For reference, the initial answer proposed a generic “detect page load”:
https://developer.mozilla.org/en/Code_snippets/On_page_load