After I have entered my Master Password, does Firefox decrypt all the logins and store them somewhere? Or does it store the Master Password itself? Is the in-session period (more) vulnerable to attack?
The password database isn't encrypted directly with your master password, instead a hash of the master password is computed and that hash is used to encrypt/decrypt the password database.
After you typed your master password, Firefox doesn't store the plaintext password anymore and instead simply use the hashed password to access the file.