fix error: 'downloading' is an invalid string label

@brade and I noticed the following message on the browser console during while testing esr78-based updates:

'downloading' is an invalid string label

This error is familiar to us because we fixed the cause already while uplifting the downloading patches (see Bugzilla 1642404). We should backport the missing portion of the patch that is now in Firefox. Alternatively, we could remove the #28885 (closed) parts of our #4234 (closed) patch and backport all of the uplifted code from Bugzilla 1642404 and Bugzilla 1642754. Those fixes are included in Firefox 79.