(UPDATED) The GWT developer plugin has been updated to works with Firefox 9, a couple of days ago.
Its not yet officially available, but can be downloaded directly from the codebase:
google-web-toolkit.googlecode.com/svn/trunk/ prebuilt folder in here is the gwt-dev-plugin.xpi, and it works like a charm :)
Previously i wrote when the plugin was updated to work with Firefox 7 and 8.
More info in the group discussion: “GWT Developer Plugin for Firefox 9”
What timing, as Mozilla is likely to release Firefox 10 this coming Tuesday. For the sake of web developers that use GWT, i hope the plugin updates can come sooner in the future.
UPDATE 2012-02-02: This thread in the google group follows plugin development for Firefox 10.