Since some time sdk/widget is deprecated and according to https://blog.mozilla.org/addons/2014/03/13/new-add-on-sdk-australis-ui-features-in-firefox-29/ sdk/ui/ should be used instead.
Unfortunately sdk/ui/frame places itself on sdk/ui/toolbar and it may be not moved to main menu. Toolbar is unacceptable for me, but sdk/widget support will be dropped in future - so I need some solution.
For reference - sdk/widget frame (with purple, fully filled progress bars) and a sdk/ui abomination with its own menu below (black, not filled progress bars).
http://i58.tinypic.com/2ns13qe.png