Sarah Grantham pointed out an interesting source for info on Firths in Derbyshire.

This site
http://homepages.rootsweb.ancestry.com/~spire/Yesterday/inde...
has extracts from Apprentice Records, Bastardy Papers, Board of Guardians, Cemetery Records, Coroner's Records, Law and Order, Red Tape, Removals, Rogues and Vagabonds, Settlement Certificates, Schools, Settlement Examinations, Wills and Administrations, Workhouse Registers

To search for people surnamed Firth, use the following (without the quotes) in google or similar search engines:
"firth site:http://homepages.rootsweb.ancestry.com/~spire/"

I see ~11 hits, going back to 1615.

Richard Thrift