That's nonsense. They did not target Firefox, because it has not made any major (architectural) changes in regards to security, since they've last targeted it.

That's nonsense. They did not target Firefox, because it has not made any major (architectural) changes in regards to security, since they've last targeted it. No major changes means that there will also likely not be any new security vulnerabilities and therefore they would almost certainly not find anything new. This does not mean that Firefox is less secure, it just means that Mozilla has not innovated much. And if you look at actual statistics, you'll see that Firefox is not behind at all. You could maybe make a separate case that this will lead to bad security in the future, as some innovation is inevitably going to be necessary, but with addon signing, E10s, WebExtensions and Servo all up and coming, you'll have a hard time to actually defend that case.