I'd say during the NeXT years, the apps on that platform were superior to the ones on Mac OS. One example: Virtuoso, by Altsys. It was a vector drawing program that was then morphed into what became Freehand 4 (the file formats were even the same). Another example: Improv by Lotus. It was later ported to Windows. In both cases, the non-NeXT version was clunky in comparison.

Also, it's not declarative UI in and of itself. It's just one of the results that dynamism, well-integrated into the language, brought. Correct me if I'm wrong but C# is still a strongly typed language (don't know anything about Firefox XUL to comment).