Epiphany is a joke of a web browser designed for the horrid GNOME 3 desktop environment. The browser is designed using the GNOME features-are-bad philosophy. The result is that has close to none and the browser lacks even the most basic features one expects from a "modern" web browser.
The browser is based on Webkit and does manage to render web pages correctly.
The browser does not let you configure a proxy specific for it since that would be considered a feature, and the GNOME developers are all against features.
You can turn plugins on and off, but you can not select which plugins you want. It's all of nothing. Being able to configure which plugins you want would be considered a feature, and the GNOME developers believe that basic features are bad because they could somehow confuse users.
There is really nothing more to say about this excuse for a browser except that if you really hate features then Epiphany is probably for you.