In fact, in my chrome setup I have disabled the PDF viewer because I prefer to download and open them in mupdf.

For PDF viewing especially, it wouldn't be impossible to hav an offering of various competing plug-ins, some perhaps offering extra features like annotations and others offering just the bare minimum.

I think firefox is held to a higher standard because "firefox user" is correlated positively with "aware of privacy issues". Those that just want something fast and easy to use might have used to use firefox instead of IE, but most of them have now moved to chrome.