about config preferences and js

I wonder if it is possible to get values of certain preferences set in about:config using javascript?

Incentive is to get value of preferences set in firefox addon i've created when user lands on addon front end. Basically, I'm trying to identify users landing on FE without asking them to login explicitly.

Yes, you can.

First, you need to know that Mozilla uses the XPCOM interfaces for the preferences system.

Three used interfaces are nsIPrefService, nsIPrefBranch and nsIPrefBranch2.

The preferences service is instantiated in the same way you instantiate any XPCOM service.

Two examples to make it clear:

// Get the root branch
var prefs = Components.classes["@mozilla.org/preferences-service;1"]
                    .getService(Components.interfaces.nsIPrefBranch);

// Get the "extensions.myext." branch
var prefs = Components.classes["@mozilla.org/preferences-service;1"]
                    .getService(Components.interfaces.nsIPrefService);
prefs = prefs.getBranch("extensions.myext.");

And there are 3 types of preferences, they're string, integer and boolean. There are six methods in nsIPrefBranch that read and write preferences: getBoolPref(), setBoolPref(), getCharPref(), setCharPref(), getIntPref() and setIntPref().

More examples on that:

// Get the "accessibility." branch
var prefs = Components.classes["@mozilla.org/preferences-service;1"]
                    .getService(Components.interfaces.nsIPrefService).getBranch("accessibility.");

// prefs is an nsIPrefBranch.
// Look in the above section for examples of getting one.
var value = prefs.getBoolPref("typeaheadfind"); // get a pref (accessibility.typeaheadfind)
prefs.setBoolPref("typeaheadfind", !value); // set a pref (accessibility.typeaheadfind)

You can also use complex types. By using nsISupportsString, that is used to handle strings in preferences, so, use this when the preference value may contain non-ASCII characters.

Example:

// prefs is an nsIPrefBranch

// Example 1: getting Unicode value
var value = prefs.getComplexValue("preference.with.non.ascii.value",
      Components.interfaces.nsISupportsString).data;

// Example 2: setting Unicode value
var str = Components.classes["@mozilla.org/supports-string;1"]
      .createInstance(Components.interfaces.nsISupportsString);
str.data = "some non-ascii text";
prefs.setComplexValue("preference.with.non.ascii.value", 
      Components.interfaces.nsISupportsString, str);

I hope you solve your doubt with this.