To be able to open some or more than one facebook accounts as well as in the Mozilla Firefox browser, then you will need an extension called CookiePie.
Here’s how to open a few or more than one facebook accounts at once in Mozilla Firefox using CookiePie:
1. Intall first add-ons CookiePie (I angap you can install the add-ons in firefox).
2. Once installed restart your firefox.
3. Open up and Log into your account first, after entering the right-click on the first facebook tab and select the menu Toggle On / Off CookiePie. so there is a cake icon on the right end.
4. Create a new tab and then do the same as No. 3 (right-click menu and select Toggle On / Off CookiePie).
5. Address facebook open again in tab number 2 and log in using your facebook account to another.
Here Video Tutorial How to Open Multiple / More Than One Account up at the same time in one browser.
This method not only special for facebook, but also allow you to open another account such as games, email and more.