In bug 814839, we hit a crash at least Mac 10.8.2 machines with at least one NV card in the WebGL conformance tests for depth_texture. To resolve it, we blocked packed_depth_stencil on NV+Mac. In bug 908905, we switched to blocklisting depth_texture instead of packed_depth_stencil on these systems. We should further reduce this so it's only blocked on <=10.8.2, and not >=10.8.3.
Is there a way to force-enable this on a OSX system?
You could disable the driver workaround pref, but that might break other things. It's pretty all-or-nothing. We don't have a pref for ignoring this specific protection.