It’s known for some time, that Chrome and Firefox will drop support for the APIs needed to use the Development Mode plugins. It’s also known that this will happen in 2014. That’s why GWT supports Super Dev Mode (Source Map based debugging in the browser’s developer tools) for some time.
According to Brian Slesinsky, GWT Dev Mode won’t work with the upcomming release of Firefox 27. His exact words are:
Mozilla has stopped exporting some C++ symbols that the Firefox plugin relies on . Therefore it’s not possible to support Development Mode in any new versions of Firefox starting with 27.
As a workaround, I am doing one last release to get the plugin working again with Firefox 24.2 (and hopefully newer point releases on the ESR track). If you wish to continue to use Development Mode on Firefox, you will need to download this version from Mozilla . For more details see the issue tracker .
It was never an officially supported API.