If Firefox start-up is slow for you its time to check if its because of your add-ons you have installed. Firefox has release list of add-ons which slowdown the experience by considerable amount, upto 74% and some of them even affect your browsing experience.
Now I am guessing that soon Firefox will have a new feature rolling which will check for these kind of extension and warn when users install it or if it is already installed. Something which IE has from a long time.
Firefox Group either should add a performance ranking system right next to Add-ons or rollout an official add-on which can benchmark any add-on. Via Download Sqaud