Detect when user resizes browser

There is a couple of ways to listens to the event triggered when a user resizes Firefox:

body.onresize
- when put an alert function for testing purpose, it alerts only once
- must be put inside loaded doc body, otherwise can't utilize 'body' var
- not valid HTML 4.0 spec

window.onresize
- alert twice
- can be put on chrome level or in document level
- valid HTML 4.0 spec

after some more tests:
- self.screen.availHeight = the whole screen height - taskbar height // not reliable
- window.innerHeight
- seems to be working accurately