In the U.S. there are 4 categories of ARFF (Airport Rescue and Fire Fighting) A,B,C & D, with A having the least capability and D having the most. For example an 'A' class airport has at least one crash truck with 500lbs of dry chemical or Halon. ICAO requirments are different, with categories ranging from 1-9, 9 being the best. Once again an example of a class 6 airport would have at least 2200 gallons (8300 ltrs) of water for mixing of foam and at least 500lbs (225 kgs) of dry chemical. The airports index can actually change at different times of day due to fire station manning, equipment maintenance, etc. My experience is with cargo operators so I'm not sure if it applies to pax carriers or not but we can operate into a class 'A' airport with up to an MD-11 domestically and a class '6' internationally using ICAO standards. I'm not sure where to find more info on line about the ICAO stuff but you can find all the info you ever wanted to know about this in the U.S. under FAR 139 subpart D. Here is a link to that section. Just type 139 in the right box and do a search, then look at sections 139.315,317, & 319.
Wow, that's longer than I thought but just copy and paste it.
Hope this helps!