[FF driver] Allow executeScript commands when a js alert box is showing 

CLA was signed

After 2.17 code was added that prohibited most actions when a javascript alert box was showing.  This made sense as in real life a user could not follow a link or toggle a checkbox when the browser was in this state.  However executing javascript commands is something a user never does.  The capability only exists for use by the testing tool.  As such there is no reason to keep prevent it from happening just because an alert is showing.  Because of this our project has been stuck on 2.17.