user_pref function

Is there a way to call the user_pref() function
dynamicaly, maybe from javascript?

The prefs.js file can be changed without restarting
via "about:config" - it works.
Why wouldn't it be possible to call e.g.
   user_pref("xpinstall.enabled", true)
from a (trusted/local) javascript?

I need to change some values frequntly, for instance
the manual/automatic certificate selection.
It would be nice to have a script on toolbar
that calls user_prefs("security.default_personal.cert", ...)

Tomas