We need to be able to have Sync and FxA activities interleaved with Gecko. For example: * Launch Fennec * Open Settings * Tap Sync. * Tap a help link. * Read page. * Hit 'back' to get back to Sync setup. That's currently not possible. This hard-blocks Bug 962646 from being resolved via "just launch Fennec". It blocks us from providing access to FxA Sync setup from within Fennec itself.

We're able to do half of this with some manifest tweaks: * Open Fennec * Settings > Firefox Account * Click a link * Hit back: goes back to the FxA wizard. * Hit back again: * Desired: back to Fennec. * Actual: drops to launcher. We cannot solve this for the classic Sync setup, due to Bug 774233 -- the above works by making each part of the app a separate task (which, incidentally, is what Chrome does). In order to complete the simplest version of the desired flow, we need to augment this by tracking a subset of stack events -- particularly when we've invoked BrowserApp -- and fake the back stack ourselves. In order to behave correctly in the broadest case, and be a generally good citizen on Android, we'd need to be able to support multiple Activity instances backed by the same abstract browser. That's a longer-term refactor, which would also enable some magic.

We have completed our launch of our new Firefox on Android. The development of the new versions use GitHub for issue tracking. If the bug report still reproduces in a current version of [Firefox on Android nightly](https://play.google.com/store/apps/details?id=org.mozilla.fenix) an issue can be reported at the [Fenix GitHub project](https://github.com/mozilla-mobile/fenix/). If you want to discuss your report please use [Mozilla's chat](https://wiki.mozilla.org/Matrix#Connect_to_Matrix) server https://chat.mozilla.org and join the [#fenix](https://chat.mozilla.org/#/room/#fenix:mozilla.org) channel.