Most people are probably aware that data they directly give to Facebook — such as “liking” a Page or updating their relationship status — may be used by Facebook to sell to advertisers. But less people may know that Facebook can also track their activities on other websites that have integrated with Facebook’s tracking technology, such as the pervasive “Like” button.The add-on, which can be installed through the usual means in Firefox, essentially “isolates” your Facebook profile from the rest of your web browsing, meaning you can still use Facebook as usual but without the off-site tracking part.
This extension helps you control more of your web activity from Facebook by isolating your identity into a separate container. This makes it harder for Facebook to track your activity on other websites via third-party cookies.
Mozilla does not collect data from your use of the Facebook Container extension.
If you use your Facebook credentials to create an account or log in using your Facebook credentials, it may not work properly and you may not be able to login. Also, because you’re logged into Facebook in the container tab, embedded Facebook comments and Like buttons in tabs outside the Facebook container tab will not work. This prevents Facebook from associating information about your activity on websites outside of Facebook to your Facebook identity. So it may look different than what you are used to seeing.
What does Facebook Container NOT protect against?1It is important to know that this extension doesn’t prevent Facebook from mishandling the data that it already has, or permitted others to obtain, about you. Facebook still will have access to everything that you do while you are on facebook.com, including your Facebook comments, photo uploads, likes, any data you share with Facebook connected apps, etc.
Source: Mozilla Blog