Putting the flags in Firefox just seems logical not "Hostile Design". Yes, there could be an easier way to turn it off, such as a menu item, but the flags need to be there first before the menu entry can exist.
The author claims to be an "IaaS engineer", surely, he can figure out how to write a firefox plugin, that can do what he wants, and use that to help non-technical users, and if it becomes popular enough will probably effect the change he wishes to see.
If you don't want new features, don't upgrade it, what in the non-sequitur is this? I get the argument that it SHOULD be OPT-IN rather than OPT-OUT, but that would require annoying pop-ups every upgrade that explains the new features and ask if you want to OPT-IN. That is more burden on the developers and will annoy more users than benefit.
If you are concerned, they do have what is called a 'changelog' that will explain all of the new features and how to switch them off if you like.
You might want some of the new features (such as TLS 1.3, WebP, some security fixes, etc), but avoid some others (such as HSTS, many new web APIs, secure contexts, AI, some CSS commands, etc), and want to keep some features that are removed in later versions (such as several settings, and many other things).
You're right, you may want some features but not all of them. That is why firefox provides the flags for you to turn features on/off. You mention that a user might wast "TLS 1.3, WebP, some security fixes, etc". I would argue that if a user knows what these are, they are capable of working out a flag.
Not all of the functions can be controlled by the flags, though.
It’s probably not too complicated, more a matter of how to expose settings to users in a way that makes sense. Every flag could be automatically turned into a better UI or menu somewhere, but then you have thousands of settings no one cares about which would be easy to use incorrectly. The stuff that shows up in context menus and settings needs to be a least a little bit curated for it to make sense. about:config isn’t exactly hard to use either (there’s an actual UI, not the code shown in the blog post).
In this case, yeah, having a single option to toggle off AI settings makes plenty of sense to curate a settings page for! But it’s probably a prioritization or product problem, not a technical issue.
Its not just that each new "feature" is unnecessarily difficult to disable, and already active-with-privacy-side-effect by the time you notice.
Most new "features" are by now covered by an existing setting and/or policy. Yet I recognize a pattern of introducing new "but did you opt out of THIS NEW thing?" or "but did you opt out of VERSION TWO of this previously rejected thing?" setting/policy. It has become unsafe to upgrade to new Firefox releases, because each one will disrespect previous user choice in another unexpected way.