It looks like we can't launch a popup from within the Trusted UI. For me, nothing happens, no error. Since it would be unrealistic to backport a platform change to all old Firefox OS versions let's change the Spartacus FxA oauth login to use redirects instead of a popup. It should be possible to re-enter Spartacus using the data-start-view body attribute: https://github.com/mozilla/webpay/blob/master/webpay/spa/templates/spa/index.html#L41
Please add STR here or mark it with [qa-] if no QA is needed.