You can use [Choosy](http://www.choosyosx.com/ "Choosy - A smarter default browser for Mac OS X") and set the rules for the URL you’ve created in Dock. Not only it works for the URL in the Dock, the rules also apply to all the links you open in Safari, Firefox, or Chrome. If you need a certain domain, or URL patterns to be opened in separate browser, then Choosy is the one you need.