The `extraPrefs` parameter is injected verbatim into the mozilla.cfg
file.
Note that the syntax is a superset of the usual prefs.js syntax. The
following procedures are of particular interest:
pref() to set a preference as if it had been toggled in about:config
defaultPref() to set the *default* value of a preference
lockPref() to set a preference & prevent further modification
clearPref() to reset a preference to its default state
Example:
```nix
tor-browser-bundle-bin.override {
extraPrefs = ''
// Increase default security level
pref("extensions.torbutton.security_slider", 2);
'';
}
```
