[tor-bugs] #24504 [Applications/Tor Browser]: Embedded WebExtensions broken in TBB 7.0.10

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#24504: Embedded WebExtensions broken in TBB 7.0.10
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     Reporter:  learningcrumb             |      Owner:  tbb-team
         Type:  defect                    |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 Embedded webextensions no longer work in TBB 7.0.10. Note that they do
 work in vanilla FF 52.5.0 and used to work fine in TBB 7.0.7.

 This breaks certain addons and is particularly inconvenient at a time when
 Mozilla is gradually forcing us to migrate to webextensions.

 Steps to reproduce:
 0. Download and unzip the attached test extension. Look at the code to
 verify it is basic and harmless.
 1. apt-get install firefox-esr
 2. Open firefox, Help / About to verify version is 52.5.0
 3. Browse to about:debugging, Load Temporary Add-on, find directory with
 test extension and select install.rdf
 4. Button appears on toolbar, title text should be 'Test'. Click it.
 5. Open Tools / Web Developer / Browser Console and verify that 3 messages
 have appeared:
 {{{
 main: started
 main: embWE should hopefully be running now
 embWE: button clicked
 }}}

 6. Now open TBB, Help / About to verify version is 7.0.10, based on FF
 52.5.0
 7. Repeat steps 3, 4 and 5 above. Note that only the first 2 messages have
 appeared. This shows that the outer extension is running but not the
 embedded webextension bit of it.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24504>
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