Recently, while I was adding keyboard navigation support to an addon, I discovered a very frustrating bug in Mozilla's Addon SDK. Basically, keyboards events (keyup, keydown, etc) were being fired twice on various elements inside a panel. Strangely, this seems to be a Linux specific bug (can't vouch for Mac though) because it isn't happening in Firefox on Windows. I've reported to the bug to Mozilla along with a testcase (Bug 707623), lets hope it get's fixed soon.
In the meantime, I found a workaround which is to call event.preventDefault() on the keyboard event, which prevents the second event from firing for some reason.
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