just choose to ignore the update. deleting Safari in the Applications folder wont entirely remove Safari its just the frontend application to the WebKit frameworks. many processes in OS X use Safari/Webkit like Dashboard, iTunes Store etc. so its always good idea to keep it up to date.
also the obligatory why have you ditched Safari so quickly for Firefox? Safari integrates very well into the system i.e. with Mail, iCal, iPhoto etc. and is a pretty decent stock browser in contrast to IE in Windows. but if you use Firefox for the features that Safari lack then fair enough use what you wish.