My feeling is that not everything has been removed and it might be interfering with your internet connection.
I have 4 Windows 7 computers. Two of them use FF ESR; the other two use Palemoon. Of the two that use FF, one is hardwired and the other uses wifi. Comcast is my ISP but I have never used one of their modems. I buy my own modem and separate router from the list of approved modems.
I've never had the issue that you are experiencing.
The fact that only one PC has a problem, lets us rule out the modem/router as the cause. So it boils down to something on your PC that is causing the problem.
Since a restore to an earlier time did not work, we'll try something different.
Download Revo Uninstaller. Run the program to install it. I always use this to remove programs rather than Windows built-in one.
Revo Uninstaller Free
Then do something weird: install the wireless USB adapter with its driver and Cisco programs.
Now open Revo and Uninstall the Cisco programs, one at a time, allowing the program to also scan the registry. When the list for removal appears, check the items in black below the red items. Do not remove any red items. If you are scared just do a few for removal then have it scan again and remove a few more. I've never had a problem with Revo but then I am very careful about what I select for removal. If I'm not sure, I let it there.