Cleanup chaos/order modes for Firefox

Summary:
  - Moves z-index rules to z-index CSS.
  - Fixes Order mode in Firefox with JS. ;_;

Test Plan: Used Order mode in FF, Safari, Chrome.

Reviewers: AnhNhan, chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5740