[Affected versions]:

• 78.0a1

[Affected platforms]:

• all

[Steps to reproduce]:

1. Open Firefox try build (https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/EpHIZw1jSPCrZJyzkbcBAw/runs/0/artifacts/public/build/target.zip)
2. Go to AMO and add the DDG Privacy Essentials add-on
3. Perform a search with DDG and check that the search code is correct ("newext")
4. Close Firefox and set user.js pref to point to main-preview (DDG is present on the engines list)
5. Open Firefox and perform a search using DDG

[Expected result]:
2. DDG is set as default search engine
5. New search codes are used

[Actual result]:
2. DDG is not set as default (the prompt disappears rapidly when page https://duckduckgo.com/app?post=1&atb=v222-6 opens)
5. DDG uses the old search codes ("ffab" etc)

What's happening here, is that in the configuration, DDG is seen as creating a new third-party search engine, just like Ecosia and other add-ons do.

The difference here, is that the DDG privacy essentials add-on opens as soon as it is installed. That switches tabs, and in the process kills the prompt for setting as the default search engine.

This doesn't block the work in bug 1635220, as we don't need to handle this case there, but we may need to consider it in future.

Moving across to WebExtensions, as fundamentally, it is a problem with the add-on manager / WebExtensions UI handling.