I know the ip addy is always the same (static) and the dial-up is not (dynamic)
Actually, this is not the case any longer. Once upon a time, most broadband connections were static IP, but these days you will be hard pressed to find a provider who will give you a static IP address. If they are willing, they will usually classify you as a commercial customer and charge you a higher rate. They do this for a variety of reasons, one of them being the one you mentioned, and another being to prevent people from eating up all the bandwidth by running public servers from their bedrooms.

what I am getting at is it a good idea for me to have a firewall as a dial-up user for when I am on for extended times?
The question should be, "Is there any good reason NOT to use a firewall?" and the answer is no. And there is every reason to have one. Go get it. Now.