According to the plugin author, GWT Development Mode won't be supported in Firefox 27+. [1] I suppose workarounds will be developing in Chrome, developing with Firefox 24 ESR perhaps, or investigating GWT Super Dev Mode. [2] [1] https://plus.google.com/+BrianSlesinsky/posts/2ZsefAgkV8j [2] http://www.gwtproject.org/articles/superdevmode.html Greg Sheremeta Red Hat, Inc. Sr. Software Engineer, RHEV Cell: 919-807-1086 gshereme@redhat.com