I'd say you're being generous calling it poor design. It's actually reverting to "default" on settings when you uncheck instead of storing individual bits and honoring those. Why not revert to opted out - OK, that may be lazy to use a single template, but that's not the way some of their other "master" options work. And I've been a FF user since it's first releases, so this isn't some Mozilla hate. And I won't be going to anything Chromium and because of inertia I may just stick to FF.
It's also crazy that I have been manually configuring away from AI since it wasn't even opt out... it was forced in. Most aren't going to do that and Mozilla knew it going in. And I've already seen those registry settings revert once. Since this control option literally should have been the first feature for AI delivered and their entire AI push has an untrustworthy stink, I'll say it again: I await a future release bumping the setting back "on". "Oopsie! you can just turn it back off or wait for the next patch" after Mozilla and their partners collect their information across millions of users that aren't paying attention.
I don't think I'm being paranoid by saying it:
opt-out rollout of every AI feature
only slogging through registry to manual opt out until now
CEO and board hell bent on monetizing and delivering features users actively do not want. I.e., enshitification
I have seen my own AI registry changes revert already once after a patch