Add Firefox 3.7alpha support (still need Windows changes).

Patch by: jat
Review by: conroy

Review at http://gwt-code-reviews.appspot.com/638801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8276 8db76d5a-ed1c-0410-87a9-c151d255dfc7