What exactly is StumbleUpon and how does it work StumbleUpon is primarily a browser plugin, a toolbar (works in IE and Firefox) that allows users to uncover and to vote new web pages, videos, new pics, news, etc. But how do these pages appear? As a result of a certain user interaction (called Stumbler) with the toolbar, you can click on the button called Stumble!, and the result will be displayed in a new b ...