I don't know anything about networking or ports or anything and am afraid of opening a security hole in my system.

I've installed Firestarter then went to shields-up and did a test.

It said that I could be pinged, so I checked off the enable ICMP filtering, but nothing else. Didn't fix the problem so I think it's my dsl modem...

I then enabled port 465 from yahoo.com, but haven't done anything else.

Anyway, now I can't "sudo aptitude update" cause it seems to be blocking port 80.

My question is... is it o.k. to uncheck the ICMP filtering box or should I leave it checked off? If I leave it checked off, how do I get aptitude update working again?

You should be able to run firestarter as root, and then start aptitude update. Look through the things happening in the firestarter gui and you should be able to find aptitude as trying to do something. Then you can whitelist aptitude.

I kind of doubt that Firestarter has anything to do with it. Besides making sure you're running as root, you might try editing your sources.list to read from an ftp server instead of the http server. I've noticed intermittent problems with the http servers lately.

Change...
deb http://ftp.us.debian.org/debian/...
deb-src http://ftp.us.debian.org/debian/...

to...
deb ftp://ftp.us.debian.org/debian/...
deb-src ftp://ftp.us.debian.org/debian/...