Global upload - fix for Firefox

Summary: Fixes T5188; see task for discussion.

Test Plan: made error condition occur with Firefox and was able to dismiss it by clicking 2x - once to focus window and then once to actually click in the window. Did this multiple times in a row with no errors.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5188

Differential Revision: https://secure.phabricator.com/D10102