Since there are Mozilla devs here, let me ask a question: Why? Why not have the browser be really solid for strictly HTML/JS processing, provide a stable exten

Since there are Mozilla devs here, let me ask a question: Why? Why not have the browser be really solid for strictly HTML/JS processing, provide a stable extension/plugin API, and have everything else be an optional add-on? Why do I need a PDF viewer built into a browser? Why an RSS feed reader built in? Why bundle in "social" features? Why not leave these to user-selected, user-loadable options?