Comment 4 for bug 875160

I also prefer the gradient separation between the primary toolbars and the title bar as they are two different bars.

In order to fix the inconsistencies you mentioned, Firefox/Thunderbird (and every other GTK2 app which uses the "dark toolbar" style, like Polly) need to migrate to GTK3, and that's not something trivial nor fixable in light-themes.

In any case, updated styling for Metacity has landed and should make this inconsistencies less prominent.