"Everything is open source" does not contradict "hermetically sealed product".
I built myself an extension. Just for myself, nobody else. It worked great, then in one of these "policy changes" I couldn't use it anymore. Just for myself. That is a betrayal of trust. If I could use it before, and now I need to hack/mod Firefox by building it on a machine with umpteen hundreds of gigabytes of storage, that is a betrayal of trust. Firefox is "demonstrably" a hermetically sealed product. As demonstrated by my lived experience.
When software auto-updates and stops working, we consider that breakage. When software auto-updates and code I wrote stops working, we say "oh, it affects just 0.01% of users." For me, the inescapable lesson is to not write code for extension eco-systems, because they're all too immature for serious use. At least as of the year 2015.
I built myself an extension. Just for myself, nobody else. It worked great, then in one of these "policy changes" I couldn't use it anymore. Just for myself. That is a betrayal of trust. If I could use it before, and now I need to hack/mod Firefox by building it on a machine with umpteen hundreds of gigabytes of storage, that is a betrayal of trust. Firefox is "demonstrably" a hermetically sealed product. As demonstrated by my lived experience.