[tbb-bugs] #21876 [Applications/Tor Browser]: e10s is not enabled on Linux (and probably OS X) by default in ESR 52 based nightlies

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#21876: e10s is not enabled on Linux (and probably OS X) by default in ESR 52 based
nightlies
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 Reporter:  gk                                   |          Owner:  tbb-
                                                 |  team
     Type:  defect                               |         Status:  new
 Priority:  Very High                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Major                                |     Resolution:
 Keywords:  ff52-esr, tbb-7.0-must-alpha,        |  Actual Points:
  TorBrowserTeam201704                           |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by mcs):

 Replying to [comment:3 cypherpunks]:
 > It's not enabled by default anywhere (in FF too), because this feature
 is not ready for GA. So, it is rolling out to separate groups of users,
 and e10srollout add-on decides when and for whom to enable e10s.

 That is correct, although for all esr or release channel Firefox users,
 e10s does get enabled by the e10srollout add-on unless the user has an
 incompatible extension installed.

 For our TB nightly builds the update channel is "default", which means
 that the e10srollout add-on does not do anything (the channel is unknown).
 See:
  https://dxr.mozilla.org/mozilla-
 esr52/source/browser/extensions/e10srollout/bootstrap.js#61
 The other part of the story is that for Firefox nightly and alpha builds,
 e10s is enabled by default but it is not for our nightly builds. Why not?
 Because RELEASE_OR_BETA is defined for our builds, which means the
 `browser.tabs.remote.autostart.2` pref is not set to `true`. See:
  https://dxr.mozilla.org/mozilla-
 esr52/source/browser/app/profile/firefox.js#1438
 One solution is to patch `browser/app/profile/firefox.js` to remove the
 `#ifndef RELEASE_OR_BETA`. I don't think we want to arrange for
 `RELEASE_OR_BETA` to be defined because that may have other undesirable
 effects.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21876#comment:4>
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