And when there is a security vulnerability which exploits webkit by just visiting a website, one has to wait till Apple releases an OS update(if it decides to acknowledge and fix it[1]), where as one on Firefox 'if Apple had allowed its own engine' could get updated via the AppStore.

What Apple has done with the trackers is commendable, but the security argument when it comes to browsers is always in favour of allowing browsers with their own engines; which apple doesn't.

[1]https://arstechnica.com/information-technology/2019/09/apple...