R1:c3635e446461

Hide oncopy <pre> in Firefox

Test Plan:
Comment `JX.DOM.remove(pre)` to better see the problem in old code.
Apply this diff and verify that the pre is not shown.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2245