Not to be confused with football/soccer firms. Firm (which in this context has exactly the same meaning as mafia) is a term used for organised criminal gangs in the United Kingdom, particularly England. Firms usually consist of white Britons, often run by one family. They are usally highly organised and keep very much underground, some firms may get corrupt police and politicions on their side, but in general they try to avoid all forms of authority.
They opperate in very similar ways to other criminal organizations like the Italian/sicilian mafia and Irish mob, like them, firms also have traditional values of respect, looking after your family and the rest of the firm and the code of silence.
Their criminal activities are mainly for financial gain, and they include; drug dealing, gun running, smuggling, extortion, armed robbery, protection rackets, kidnapping, the list goes on. Some larger firms opperate on an international basis, you will find quite a few British firms operating in places like the Costa Del Sol in Spain, and in other parts of Europe too.
Possibly the most well known of all Britains crime firms are, The Kray twins, refered to as "The Firm", they were one of the most notorious gangs in Londons east end in the 60s.