Product and UX gave Engineering the go-ahead to enable all update2.* branch prefs on Beta.

Why just early beta?

Isn't that the way we normally roll features out? We enable on early Beta so we don't break Release QA if the feature doesn't end up shipping. We meet again at the end of the early Beta period to decide if we the feature should ride to Release. See bug 1613869. That said, if you'd prefer I enable this for all of Beta, I can certainly do that instead.

I think the choice is just between early beta and then land a further patch for release if beta goes well, or enable for release and then land a disabling patch if beta goes wrong.

Indeed, so I think all of beta would be fine here, since we have our go-no-go for release during late-beta as well.

In that case, I'll go with Marco's approach of enabling for all of Release and then disabling later if need be. Afaict there's no combination of release flags that covers Nightly, late Beta, and Dev Edition. From https://wiki.mozilla.org/Platform/Channel-specific_build_defines#Usage:

(Also note there is intentionally no single variable covering Beta, as late Beta becomes/is Release.)

I won't dupe this to bug 1665048 since that tracks other touch-up items we should do once we get Release approval.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=6d586b5f02c080805a8c868a70d725f4c970cdf5

https://hg.mozilla.org/mozilla-central/rev/8e41ebb110cf