TBB prefs are set as user_prefs (may cause exceptions/settings errors?)

All of the prefs we change in TBB appear to be set as user prefs. This makes it hard for me in Torbutton to tell if the user has changed a value.

We also have occasional reports from people that some prefs seem inexplicably set (such as form fill options). Since user_prefs() sets a user value, it probably causes Firefox to emit observer events for each preference we change in this way. If one of them has an exception because the observer for that pref is buggy and/or tries to touch some not-yet-initialized piece of Firefox at startup, that could easily cause some prefs to fail to apply randomly.