This had me wondering if it was my Admin Menus Fixed plugin but disabling it and Ozh' Admin Drop Down Menu made no difference. In Firefox 4, only the Dashboard, if the Stats widget is showing, gets messed up. It's fine up until the Stats widget fails to load and then it horizontally compresses the Dashboard to about half the width and centers it; with the Google Analytics graph exceeding the right edge of its widget. With the two plugins, above, activated, the dashboard remains full width but ends up going off the right edge, with no scroll bar to get to the hidden section.
In Firefox 3.6.15, I get a flash glimpse of the Dashboard and then the little smiley in the upper left on a blank screen that people are seeing from the WordPress.com Stats plugin replaces it. Dashboard never comes back unless you close the browser and re-login after restart. Good thing I've got FF4 or I couldn't get anywhere in the Admin screens.
dorjee