Honestly, I've been a firefighter for 5 years now in Baltimore City, and I couldn't imagine a better job. The tangible feeling of helping out the community does a lot to keep your morale up day in and day out, good days and bad. In my opinion, neither job is more or less dangerous; on the fireground, most injuries are not burns, despite what the movies would have you believe, I have been put off duty for thrown shoulders, busted knees, and burns. I would say the majority of the injuries are joint injuries, simply because crawling around blind in an unfamiliar environment while wearing 80 pounds of gear lends itself to tumbles down rickety stairs, falls through weakened floors, etc. Also, when adrenaline kicks in, unless you watch yourself, it's not difficult to injure yourself and not realize it.
Regarding the whole "lounging around until a call comes in" mentality: With the exception of a few "old school" departments out there, most of your time will be taken up with building inspections, hydrant inspections, maintaining certifications through coursework, and general public relations stuff. Calls will be interspersed throughout this.
With the increased reliance on the Fire Service for EMS responses, 90% of your calls will be medic assists and car accidents. All in all, pretty busy, but not unbearable.
On the flip side there is generally less competition for the police officer jobs and the pay and benefits are generally better. (15-20% higher pay for police in many jurisdictions.)
Good luck in your career choice.