I notice the difference of how much computer resources Firefox takes up on an older 7 yo computer, 2 GHz 448 MB ram, XP. Firefox takes 3-5 minutes to fully load with 3 small add-ons, a few saved tabs.
Safari in comparison took 18 seconds, Thunderbird about 25-30 secs. Ridiculous huh? I didn't realise how demanding Firefox is until I used this older computer.
Edit: I suspect the saving of tabs when closing FF is what makes the FF take so long to restart up & settle.