Significant changes have probably been made to the implementation since the NSPlugin implementation was dropped; the protocol may have become dependent on ActiveX-specific features/settings.

I don't know enough of the technical details to fully follow the discussion, but it looks like support might have been added to Firefox ("Status: RESOLVED FIXED"): https://bugzilla.mozilla.org/show_bug.cgi?id=478839

This commit definitely makes it look like something has happened: https://hg.mozilla.org/releases/mozilla-1.9.1/rev/653b75a8f4...