Hackers don't really bother to try to gain access to a 56K modem.
They KNOW, it's not worth the effort.
Especially since your IP address does/can change everytime you connect.
I used to have 56K dial up access connecting at 48.9Kbps with a shut down company called Barrie Connex. Look Communications took over.
Now, I'm on High Speed DSL with Bell Sympatico.ca.
A firewall is not really worth it when it comes to dial-up access.
Plus IP addresses that are assigned to dial-up users are in a COMPLETELY different range than those with Cable/DSL connections.A hacker knows this.... and will not bother with your connection.
There are FAR too many fat juicy unprotected Cable/DSL connections to go after.
My friend, I would have to say that you are pretty safe.
I've had dial-up for years without a firewall and not ONCE was I ever attacked. I know a hell of a lot about computers...the apps, the core hardware..ect.
Of course... you can NEVER have too much protection. But investing in one or getting one of those freebie ones off the net is not really worth it unless you have a dedicated 24/7 dial-up/ISDN connection.
Hope this helps?
The decision is yours.