Firefox has been a low-priority target for a couple years due to its waning user-base. In fact, Firefox wasn't even at Pwn2Own 2016 because hackers didn't think it was worth their time[0].
Hopefully with Quantum and a resurge in popularity, it'll become a target of white-hat hackers again.
[0] http://www.eweek.com/security/pwn2own-hacking-contest-return...