In general, blocking userspace from installing extensions and otherwise running malicious code stops FF from being exploited. Blocking all of these options blocks the OS from bad behaviour, especially where the user may expect a new computer/phone to have the default behaviour.
The vendors of phones and personal computers seem to have an interest in interfering with users' internet access; perhaps it is a good decision that Firefox does not let them.
So no, actual Mozilla strategy does NOT work to stop malware's on Windows nor commercial OEMs customization, as a matter of fact made only life of pro users and admin harder and open the door for less safe setup (for instance extensions added via homeManager may not get updated by FF).
This doesn't change that a sanctioned API invites abuse far more readily than reverse-engineering, especially when spyware is less frequently updated than Firefox itself. That there isn't a sanctioned API, and that the de facto API can and does change every version, is an advantage for Firefox against potential attackers.
If this is something you are sorely lacking, Firefox has also been straightforward to modify and compile, in my experience. You can always share a patch and enjoy these features as part of a smaller community, without compromising the userbase as a whole.
End users are adult, not child, and developers are others adults with ZERO right on their software's user.
If people want safer systems better learn to avoid commercial software, nothing else can help them.
Perhaps a more fine-grained permission model is needed for cross-application changes, but I can't think of anybody actively working on it in an OS.
[AddOns]
ensure-ffpkg AddonName
[Themes]
set-default ThemeName
install ThemeA, ThemeB
[Settings]
key:val
...
and have the AddonName downloaded with GNUPG signature check from an official Mozilla repo is by far more save that demand using interactive GUIs. Simply ask at startup to accept "potentially dangerous" extensions if you specify a local .xpi file it the same.
What you describe is the classic Windows approach that have proved enough to be ineffective and only useful for commercial practice. Mozilla is formally a foundation and Firefox is formally a FOSS project...
It's a pain, but I can see the compromise. If it makes the average Firefox user more vulnerable, there's definitely a case to protect them, even at the expense of its more capable users.