Got the development mode plugin to compile for Firefox 15 using xulrunner-15.0b6 on 64-bit Linux.

Review at http://gwt-code-reviews.appspot.com/1816803

Review by: acleung@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11244 8db76d5a-ed1c-0410-87a9-c151d255dfc7