Bug 843500: Fix race condition in firefox tabs code between concurrent module instances.
Revert "Merge pull request #842 from jsantell/Q-all-790195" a=@gozala

This reverts commit ef7e1c3a7d2f758496b473b77f251afc1d7e599c, reversing
changes made to 424f66c3663d8d507407952ac8586ab4bc809396.
Merge pull request #838 from ochameau/tabevents-race

Bug 843500: Fix race condition in firefox tabs code between concurrent module instances. r=mossop