While our current focus is on activities around MozActivate (Rust, WebVR, etc), I really like this idea. Firefox 57 will be big, and setting a big milestone in our journey.
Having launch parties for Firefox 57 would be good way to make people aware that Firefox has changed a lot in the past few years as well as, I think, it would be a good a good entry point for future contributors. By then we will definitely have the Nightly MozActivate activity (probably in v3 or so ) so this could easily be coupled with a call-to-action “install Firefox Nightly, you will get all of that and you can help us to get even better with using Nightly”.
We might even want to consider having a MozActivate activity just for that (maybe with content leading up to the release), even though this is would not be a continuous activity.
What do others think?