Do you any source to back that up? This is the first time I hear something like that.

It's well known in the information security community for years. But here's a recent article on grading software security like how we grade cars:

https://theintercept.com/2016/07/29/a-famed-hacker-is-gradin...

They give one example (with a graph) of the security mitigations that are in popular web browsers:

https://prod01-cdn07.cdn.firstlook.org/wp-uploads/sites/1/20...

Firefox scored below Chrome and Safari in several areas: address space layout randomization (ASLR), heap protection, stack guards, fortified source. And I've even seen infosec people saying that Firefox has some of these, but doesn't even enable them on builds for some platforms!