UPDATED 2011-12-14: Prebuilt folder now contains Firefox 8 version of the GWT developer plugin.
Since Firefox 7 was released, i have defered upgrading for one simple reason: At work I am dependent on GWT Developer plugin.
Today I found the ready-to-release plugin which actually works with Firefox 7:
http://google-web-toolkit.googlecode.com/svn/trunk/plugins/xpcom/prebuilt/ in here is the gwt-dev-plugin.xpi, and it works like a charm :)
(In firefox, it asks you to download. And then you can drag the downloaded .xpi back into the browser to install. Why Firefox will not instantly recognize the .xpi file is kind of strange)