Or just local antivirus. There is a shocking amount of user level firewalls that break the web really badly.

Why would a standard browser trust that certificate?

It's installed by the ISP's setup program which they tell everyone is mandatory (otherwise they won't get the adware kickbacks) and the techs are discouraged from skipping.

How often that does even happen? Many people just get a router (w/modem) in the mail which they just connect to with their devices. No software is being installed.

100% of Comcast and Verizon installs in my experience. RCN offered but wasn't pushy and they seem to have stopped.