Many airlines allow passengers to buy in-flight internet access that is often partly or entirely free for premium cabin passengers and sometimes for elites.
You often need to use your PNR, e-ticket number, or seat + name to get the complimentary access. Some Finnair cabin crew members thought that it was a good idea to use the in-flight internet using passenger information and are now in hot water with the airline.
Here’s statement that Finnair has released for the Finnish media (Google translate):
It is true that, contrary to our instructions, some cabin crew members have used an on-board network connection for customers only. It is, of course, absolutely forbidden.
We are conducting a hearing with all staff members who have committed the abuses. This is a prohibited activity that has caused our customers harm and financial damage to us. In addition, the trust that exists between us has been violated. If a person is guilty of such blatant misconduct, it will result in either a written warning or, in rare cases, even termination.
Finnair allows business class passengers to use the internet free for an hour. You can usually request extra codes from the cabin crew members that pass those generally without a fuss based on my personal experience. Finnair Plus Platinum and Lumo members have entirely free internet access for the duration of all long-haul flights.
Some crew members perhaps thought that it was a good idea to use passenger information to access the internet for free without realizing that this scheme would eventually be exposed when customers would complain about not being able to use it.
This misuse of internet applies to both outsourced cabin crew that mainly operates flight to/from Asia and Helsinki-based ones.
I would say that firing someone purely over this is an overreaction. Giving a written warning is more appropriate.