The Internet Bad Boys seldom rest. New ways to attack your computer, steal your passwords, empty your bank accounts, etc. etc. keep popping up.
Firefox, along with Microsoft, Apple, Adobe, etc. etc. keep cranking out fixes to protect you from the latest attacks.
When you don't keep up to date, it's like leaving an open door to your computer for the bad guys to get into it.
So, even if you think it's a pain, all the experts tell us to keep up with the updates or sooner or later something bad will happen to our computers.