Steps to reproduce the bug:
unzip the archive and open bug.html
click the 'form' button
type any text inside
click 'check for bug' button, do NOT hit 'enter' key.
a message box appears containing the text you typed
click ok, another box appears with the same text
the message box appeared twice but it's invoked differently:
the form disappears
click 'form' again, the form appears
type any text but something else than the first time
click 'check for bug' (not 'enter'), a message box with the text appears
click 'ok', another message box appears but it contains the text you typed the first time, not the text that is in the edit box
my version of firefox is 3.6.6, the test was performed in safe mode