We currently disallow overriding builtin engines by a webextension. However, we rely on the engine name to do this. We should consider enhancing this. One idea might be to detect exact domain matches: given an builtin engine using test.com if name matches or domain matches /www.?test.com/ use builtin engine This would still allow subdomain.test.com to be added by an extension since we do not have a builtin that specifically uses the subdomain. An example of this in real world would be a Smile extension adding smile.amazon.com when we only build in the default amazon.com. (I'm also assuming verifying domain owner for such an extension).

@mkaply, what do you think of this?

I don't think we're trying to completely prevent people from installing other engines for our built-in engine domains. There are legitimate reasons for other search engines to those domains (safe search comes to mind). DuckDuckgo also has a ton of other options for adding search engines. See: https://mycroftproject.com/google-search-plugins.html