Embedded WebExtensions broken in TBB 7.0.10

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.

Trac:
Username: learningcrumb