Remove inner spacing on mozilla-based buttons

Summary: Fixes T6644

Test Plan:
Tested buttons in FF and Chrome

Before:
{F238749}

After:
{F238748}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6644

Differential Revision: https://secure.phabricator.com/D10903