Event just before loading a page

Ortiz <[email protected]>
Newsgroups gmane.comp.mozilla.devel.xpfe
Organization http://groups.google.com
Message-ID <6c20377a-cda3-4259-b155-ceca9c2c28a1@g12g2000yqd.googlegroups.com> 
Hi there!

I would like to detect some urls in order to redirect them to another
one. I'm looking for the right event then, a kind of preload event
that executes just after click or press enter, and just before request
the page.

Is there any event like that? If not... is there any way to get it?
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