_ff-addon-snippet-MasterPasswordPrompt - This snippet checks to see if master password exists. If it does it prompts user to enter it and if they get it right returns true else fail. If no master password exists then returns true.

_ff-addon-snippet-MasterPasswordPrompt.js

var me = Services.wm.getMostRecentWindow(null);
var tokenDB = Cc['@mozilla.org/security/pk11tokendb;1'].getService(Ci.nsIPK11TokenDB);
var master_password = tokenDB.getInternalKeyToken();

var master_password_authenticated;
try {
    master_password_authenticated = Components.isSuccessCode(token.login(false)); //login argument is false meaning that if the user had previously entered their master password it will not prompt them now AND if they had not entered it before and it is still valid for the session it will NOT prompt. IF you set it to true then regardless if the user entered it in before or not it will force them to enter it again
} catch (ex) {
    master_password_authenticated = false;
}

me.alert('master passsword pass? = ' + master_password_authenticated);

README

Rev1

Done with master pw intact

Rev2

Done with master pw intact.
Tested when there is no master pw and it passes fine. If there is no master pw then if force is false or true it doesnt matter it always returns successfully.
Using login function seen at MXR - nsIPK11Token.idl and MXR - nsPK11TokenDB.cpp