You can have multiple profiles on Firefox, but you can't password protect them.

To create a new profile (assuming you are on Windows) go to C:\Program Files\Mozilla Firefox from a command line.

Type firefox -profilemanager

Select "Create Profile" to create a new profile.

Once you have more than one profile, it will always startup with the profile manager so you can select a different profile.

Can you be more specific on the social sites problems you are seeing?

Thanks