Firefox uses 100% CPU when loading tabs

I was asleep and left my laptop on with Firefox running over night when the fan woke me up. My laptop sounded like it was going to explode. Then it shut down and the BIOS displayed a message saying my computer was shut down because it exceeded a certain temperature. Why is Firefox such a piece of shit?

I use firefox because it respects my privacy. Also, I like its tabbing style better than chrome's.