Fix changeset layout in Modern Firefox

Summary: Looks like a rule for old Firefox is causing layout issues in new Firefox. Prefer new Firefox. Fixes T4987

Test Plan:
Tested the current version of Firefox.

Bug:

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4987

Differential Revision: https://secure.phabricator.com/D9240