After travelling back in Firefox history, JavaScript won't run

Set an empty function to be called on window.onunload:

window.onunload = function(){};

e.g.

<html><body>
<script type="text/javascript">
  window.onload = function() { alert('window.onload alert'); };
  window.onunload = function(){};
  alert('inline alert');
</script>
<a href="1.html">Click Me!</a>
</body></html>

Source: http://www.firefoxanswer.com/firefox/672-firefoxanswer.html (Archived Version)

When I use the back button on Firefox to reach a previously visited page, scripts on that page won't run again.

That's correct and that's a good thing.

When you hit a link in Firefox (and Safari, and Opera), it does not immediately destroy your page to go onto the next one. It keeps the page intact, merely hiding it from view. Should you hit the back button, it will then bring the old page back into view, without having to load the document again; this is much faster, resulting in smoother back/forward page transitions for the user.

This feature is called the bfcache.

Any content you added to the page during the user's previous load and use of it will still be there. Any event handlers you attached to page elements will still be attached. Any timeouts/intervals you set will still be active. So there's rarely any reason you need to know that you have been hidden and re-shown. It would be wrong to call onload or inline script code again, because any binding and content generation you did in that function would be executing a second time over the same content, with potentially disastrous results. (eg. document.write in inline script would totally destroy the page.)

The reason writing to window.onunload has an effect is that the browsers that implement bfcache have decided that — for compatibility with pages that really do need to know when they're being discarded — any page that declares an interest in knowing when onunload occurs will cause the bfcache to be disabled. That page will be loaded fresh when you go back to it, instead of fetched from the bfcache.

So if you set window.onunload= function() {};, what you're actually doing is deliberately breaking the bfcache. This will result in your pages being slow to navigate, and should not be used except as a last resort.

If you do need to know when the user leaves or comes back to your page, without messing up the bfcache, you can trap the onpageshow and onpagehide events instead:

window.onload=window.onpageshow= function() {
    alert('Hello!');
};

You can check the persisted property of the pageshow event. It is set to false on initial page load. When page is loaded from cache it is set to true.

window.onpageshow = function(event) {
    if (event.persisted) {
        alert("From bfcache");
    }
};

For some reason jQuery does not have this property in the event. You can find it from original event though.

$(window).bind("pageshow", function(event) {
    if (event.originalEvent.persisted) {
        alert("From bfcache");
    }
});

Wire in an "onunload" event that does nothing:

<html><body>
<script type="text/javascript">
  window.onload = function() { alert('window.onload alert'); };
  window.onunload = function(){}; 
  alert('inline alert');
</script>
<a href="1.html">Click Me!</a>
</body></html>

After travelling back in Firefox history, JavaScript won't run

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