STR: In Customize, un-toggle 'Use Firefox Developer Edition Theme', save customization. Re-open Customize, and turn on the same pref. The Developer Edition theme is not restored after saving customization the second time.
Followed STR and I'm not seeing this locally so I have a few questions to try and track this down: Does the button state change as if the theme was applied when clicking it the second time? Do you have any other themes applied or have Firebug 3 installed? Can you reproduce on a clean profile?
We discussed this and it isn't reproducible at the moment. Feel free to reopen if you can figure out what was happening here.