The sentence should read “is” because the subject is “a group”, as in one singular group. A good way to test in situations like that is to take out the extra parts of the sentence. Find the subject and the verb, and take out everything else. Then it becomes very easy to see what the answer should be:
”A group of firefighters (is or are) ready to respond from their new firehouse.”
So, eliminating the excess leaves:
A group (is or are). The answer has become pretty simple now, right?
Edit: Of course @Facade said the same thing in like one sentence and beat me to the punch. I hope it’s tasty punch @Facade! ;-)