When you define a document.onclick before the page has loaded completely and you use the Netscape 4-required captureEvents method, Mozilla fires the event handler twice on every click. This is apparently a very old bug.
Test page. Workaround is not included.
Reported by Matthieu Haller.
Mozilla | Reported on 28 October 2004.
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1 Posted by Martin Ng on 12 October 2006 | Permalink
Use of captureEvents() is deprecated, see bug 330494:
https://bugzilla.mozilla.org/show_bug.cgi?id=330494