The claim is correct. The security of IE9+, Edge and Chrome is far superior than Firefox. As Geohot said, writing an exploit for Firefox is similar to a harder CTF challenge (done in an day or two). While IE or Chrome Exploits cost a lot. I can back it up with references, but https://www.zerodium.com/program.html shows a pretty clear picture (Scroll down to Payout Ranges). It will take FF like 3-5 years until they have a sandbox as good as the other Browsers, sadly.