Open the following script in Fx, click Test. So far, so good. Click Reset then test again. Two alerts?! Click reset once more and it's fine again.
If you trace the events in firebug in a correct test run the following happens: onclick() calls Test().
On the second run when it alerts twice, the first call is Test(), called by onclick(), called by handle(), which is a jQuery function, a jQuery function to 'Handle the second event of a trigger'. That's as far as my diagnosis has gone.
Works fine with IE.
// shouldn't be required but unbind() doesn't work set in HTML attributes
<button id='testButton' type='button' onclick='Test()'>Test</button>
<button type='button' onclick='Reset()'>Reset</button>