It sounds like you have two issues going on.
I think you should Tackle your software issues first before trying to resolve your service problems.
Here we can troubleshoot your connection problems related to service such as signals etc. For your software problems go to »Software
this would include Firefox, Norton, etc.
This is OT here but To end the problem with firefox go into your Task Manager and kill Firefox.exe
If you can get into firefox then go to »Comcast High Speed Internet FAQ
»Viewing the Modem's Signal Levels, Firmware Version & Log Entries
and post your signal levels.