All these could be done by the 'MockHelper', because you need to clean this objects at the end of the tests in `suiteTeardown`(currenty window.FxaModuleErrorOverlay will not be the right one anymore due to we are modifying the global!).

However, for l10n you can use something like:
https://github.com/borjasalguero/gaia/commit/f7f3cc2523946d5c5ba197dbfff82f632c7ae959#diff-b580022910a7b137d2dee789438e93b0R32

And restore it at:
https://github.com/borjasalguero/gaia/commit/f7f3cc2523946d5c5ba197dbfff82f632c7ae959#diff-b580022910a7b137d2dee789438e93b0R48