security.sandbox.content.force-namespace are all accessed once per content process start.
We should cache these values.
The last one is easy; it's just a bool pref so we can just move it to staticprefs and have done with it (though this does make it accessible in about:config; that seems a reasonable trade-off...). It also looks like this pref has had the value
false even on nightly for the past 2 years, so perhaps we don't need it anymore or it could be an ifdef?
The two other prefs are more annoying, because they're strings, and in any case they should probably be cached into the parsed (array?) data structures.