If the user installs a Firefox update and then all their HTTPS connections stop working, most users will blame Firefox unless the error messages are very specific about Lenovo's involvement, which simple blacklisting won't do. Users who follow technical news and understand the problem will already have removed the certificate manually (and removed the proxy).

I doubt it. Most non-techie users will probably not get around to installing another browser, so seeing that neither bank nor gmail nor facebook works, they'll blame their ISP.

But most people already have IE installed!