Bring up the preferences dialog. Hit the "return" key. Nothing happens. Hit the "escape" key. Nothing happens. In both cases, the dialog should be dismissed. If you hit the tab key, nothing comes into focus. Ideally, the selected panel's item in the list should get focus then, and subsequent tabs should enumerate all of the focusable items in the panel. For somebody accustomed to bringing up a dialog, and then canceling it with the escape key (or command-period on the Mac) this can be very frustrating.
*** This bug has been marked as a duplicate of 58853 ***
Preferences navigation meta bug: bug 18575 BTW, I got this info from the brand new Access Bug Radar http://www.mozilla.org/projects/ui/accessibility/access-radar.html