Alright where to start with this... First off, there are currently two different "Medic" programs in the army. Not sure which one you were looking at, but I am guessing just 68w program. This is a 16 week AIT program, takes you through EMT-2011 for the first 8 weeks, and than the last 8 weeks focus on Combat type medicine. I believe still there is the fast track program still available for people that already have there EMT prior to enlisting (As long as they are current and in good standing with NREMT).
Now with what you would be doing in the national guard, forget everything you have seen on tv with all the stupid commercials. Yeah that is not going to happen. So depending on the unit you are in, you could be doing medical readiness for soldiers (physicals), or supporting live fire ranges, or watching units do training and just providing medical support. This literally all depends on the unit.
The other program is the Flight Paramedic Program (F3), which you will not get into right away, especially in the national guard.
Now regarding the infantry comment. When you are deployed it depends on the unit you are in. If you are in a infantry unit, your primary role will be a soldier first, medic second. This comes from the TCCC Guidelines.
Firefighting- currently the 12m is still available as an MOS, now depending on your state, who knows if your state currently has a firefighting unit. If you go the 12m route you will get a lot of your fire certs. But remember depending on your department you want to work for, they still may put your through a fire academy.
Send me a PM with your specific questions your have, also
http://emtlife.com/forums/military-tactical-wilderness-ems.25/ there is a lot of threads that probably can answer most of your questions. Luno and I have answered many of the questions.