Use arguments[0].target instead of event.target. Most browsers support the event global variable, but firefox does not. arguments[0] contains all the same properties and functions, but is supported by all browsers.
As a side note: indices in the arguments array can be changed by your own code. In my current project arguments[0] contains all of my controller functions for some reason, while the global event methods and properties are contained in arguments[1]. Not entirely sure why this is, but you might need to do some debugging on the arguments array to see what values it contains and adjust accordingly. More often than not however, arguments[0] should work.