In many cases, the difference between a bug and an attack vector lies in the closed source areas.
This is going to be the case automating attack detection against most programs where a portion is obscured.
However, Firefox also needs to use the closed source OS when running on Windows or macOS.
There are also WebKit-based Linux browsers, which obviously do not use closed-source OS interfaces.
My pessimistic guess on reasoning is that they suspected Firefox to have more tech debt.
>In many cases, the difference between a bug and an attack vector lies in the closed source areas.
You say many cases, let's see some examples in Safari.