How to revert the intial decision for not saving username/password in OS X Firefox? A couple of times I clicked NO (or closed the pop-up) on Firefox suggestion to save username/password for a particular form.  

How to revert this decision and force Firefox to ask again.
If you go to the Security tab of Firefox preferences, to the right of the checkbox labeled “Remember passwords for sites” there will be an “Exceptions…” button. This will show a dialog from which you can select and remove the websites you told Firefox to never save passwords for.