ENYO-345: Utilize zIndex instead of z-index property name in Firefox.

### Issue
There is a bug in Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=1083457) where utilizing the CSS property name for z-index `z-index` does not have any effect when attempting to change the z-index value of a node. The net effect is that any `enyo.Popup` and its derivatives would not have the proper z-index value applied, leading to the control unexpectedly appearing beneath the scrim. The scrim does not suffer from this problem, as the z-index value is applied prior to its generation, which goes through a different style application path. This used to work in 2.4 because our implementation of style application was different and we were not utilizing the style object accessor technique that was implemented for 2.5.

### Fix
We make an exception for the case of Firefox and its variants and set the value utilizing the style property name `zIndex`.

### Note
We will want to remove this fix once the bug has been resolved in Firefox and its variants.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>