The problem is how do you distinguish plugins installed through the browser versus not: at the end of the day, this falls into the typical DRM problem of how do

The problem is how do you distinguish plugins installed through the browser versus not: at the end of the day, this falls into the typical DRM problem of how do you enforce restrictions (in this case, what can install plugins). The direction browsers are all taking is requiring them to be signed by the browser vendor, effectively making them gatekeepers (and you can't have a preference to disable it, because if you did you could just install a plugin by disabling the preference (i.e., edit the config file) and then install the plugin normally). That really sucks too, sadly.