Funny enough, just a few posts above this one on the front page is another blog post entitled "Want a killer product? Become more opinionated". Found that to be a nice coincidence.

I think allowing customization of the UI is a great feature set to provide, and it aligns quite well with the imagine Mozilla has set up for itself as an organization. Though, I can see how the teams behind these decisions would want to stray away from having their 'product identity' be something you can easily turn off.