Firefox was started when Mozilla (Communicator) grew too large. Let's put everything in plugins they said! Do even 5% of users need dev tools? History repeating itself.
I've seen not-terribly technical people— or at least people who'd never otherwise have firebug installed— go and edit braindamaged forms to get a webpage working right. Having it built in is practically empowering.
Not anymore, judging by binary size and lines of code.