Sure, paranoia level isn't limited 😅
I used the other image (source code here: https://github.com/crazy-max/docker-firefox-syncserver)
mainly because I think the official repo just got a good dockerfile recently. Back when I was setting things up the mozilla dockerfile was still based on Debian I think (huge image size).
I agree that it may be safer to build own images, but it can also have the disadvantage of an unfixed system.