FiremenJobs.com looks to me to be one of the best firefighter job boards on the web. When you visit the site you are offered a firefighter job search box on the left hand side. Enter a search and it will return a couple of jobs for free.
The site offers a premium subscription option that looks to be fairly inexpensive at under $10 per month.
The site claims to have 643 active firefighter and EMS jobs listed as of Saturday March 25th 2006. I cannot vouch for the timeliness of the information on the site. I have no affiliation with the site owners.