At the beginning of this thread, the main dev states interest in migrating to WebExtensions, then cites all the bugs that prevent that. Towards the end of the thread, an extension dev (`nt1m`) states that the APIs have been approved, and simply need implementation in Firefox. There's definitely a chance they won't make it into 57, but the rapid-release cycle will help them land in a release not too long after 57.

If TST is essential to your workflow, you may wish to consider using 52 ESR until these APIs are finished in mainline, or contribute implementations of the APIs (time permitting, of course).