Geoff & londonfireman-
No, you do not need citizenship in order to work as a firefighter here in the US.
I know of two depts in south east Michigan that require citizenship, but the rest do not. FDNY requires citizenship by time of hire (not by time of application), and many depts in colorado require citizenship as well. These are exceptions, most places only require that you be legally employable in the US.
I hold a british passport, a New Zealand passport, and a green card, and I've been a firefighter here for six years (last 3.5 full time).
Your training will not transfer, but experience will help to impress the board during an interview.
The hardest part will be getting residency. Do not expect a depatment to pass over other applicants, hire you, and assist you in getting a green card- it won't happen. You need the residency before you can even apply.
Other options:
Get a job with a multi-national company, and get a transfer to the US. This could result in residency.
Marry some super-hot, super-rich american lady. That way, you'll get citizenship, won't even have to work, and we'll all have a cool place to hang out and have parties at. (if you manage to achieve this, please let me know how)
See if you can get on one of the "firefighter trade" things, where a US ff goes to the UK, and a UK ff goes to the US. That way, you could at least see if you'd like it here, and make lots of contacts.
Any other questions? Please feel free to send me a PM (that's a Personal Message, not Mr. Blair- you can keep him)