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.
I was viewing https://wiki.mozilla.org/Security/Sandbox and it appears that Firefox on Linux now has some kind of sandboxing. I presume it's not as good as Chrome but I'm sure it's raised the security level significantly. Realistically, how much danger am I in by using Firefox?
