I know that gmail allows you to differentiate mails If I append a + when subscribing to a mailing list and filter it accordingly in the web client. But I was wondering if it was possible for when I check mail through pop3 to "login in" into firstname.lastname@example.org as a way to only get those mails?
No, it is not possible I'm afraid.
The reason for this is that it is the part without the + that is your actual Google user ID.
The + and whatever is after it is simply a convenient shortcut (that is allowed in several email systems). It is only used for filtering after you have authenticated (logged in) to the system.
What might help you is to add an automatic filter to GMail to move those emails into a folder then you can quickly see only those emails even from a non-Gmail interface such as Thunderbird or Outlook.