Does not happen with the default preferences dialog.
Created attachment 624470 [details] [diff] [review]
v.1 Port change from bug 730989
The pref dialog changed while they were being ported: https://hg.mozilla.org/mozilla-central/diff/68ee19a92287/browser/components/preferences/sync.js (bug 730989)
I verify fixed this bug, tested at Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120621 Firefox/15.0a2.