Update devmode Firefox plugin to support Firefox 17 on 64-bit Linux.
(Other platforms coming soon.)

Review at http://gwt-code-reviews.appspot.com/1870803

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11386 8db76d5a-ed1c-0410-87a9-c151d255dfc7