What exactly are you tring to acheive?

If you want to know when the tab in which your webpage is loaded is closed, then window.onunload should help you.

If you want to know when another webpage is closed, you cannot do this.

UPDATE:

You said that you want to know when the user closes the browser or tab. This is not possible.

But for your purpose (getting feedback), I think all you need is to differentiate whether the user is navigating to a link in your page, or whether the user is typing another URL(or by clicking a favorite).

I think for your requirement, whether the user closes the browser, or whether he types another URL, is the same - the user is navigating away from your site, and at that time you say you want to collect feedback.

This can be done in javascript.

For all the clicks in your page that
might lead to a page refresh
(hyperlinks, buttons,...), set a flag.

In window.onunload, check whether
this flag is set.

- If it is set, then
the user has clicked a link in your
page, do nothing.

- If the flag is not
set then the user is navigating away,
time to collect feedback.

Let me know if this would work.

PS: Note that popups/any distractions during window.unload can be very annoying.
I understand that this probably is the requirements given to you. But if possible, try other mechanisms to collect (voluntary) feedback from the user.