The firefighters should be offered a one-off, inflation-busting £5k pay rise in exchange for a permanent no-strike contract and an agreement to modernise their working practices.
If they don't agree, all firefighters involved in the strike should be sacked (a la Reagan and the US air traffic controllers). The fire service should then be turned into a part of the armed forces. If they want to re-apply for their jobs, the firefighters will have to join HM's finest. In the interim, the fire service will be a detachment for the armed forces and the Country will just have to get used to a reduced level of fire and rescue service.