[Environment:]

77.0a1 20200408033650 / Windows 10

[Steps:]

1. Install Nightly ln-10 en_GB.
2. Create a new profile & don't start it.
3. Create an user.js in the profile directory setting region to GB: user_pref("browser.search.region", "GB");
4. Start the localized en-GB version with the profile created.
5. Observe the list of engines displayed
6. Go to about:config and set browser.search.modernConfig to false.
7. Restart browser and recheck the search engines.

[Actual Result:]

Twitter is missing in the modern configuration while compared with the old configuration.

[Expected Result:]

Twitter is available.

[Note:]

Search Engine devtools doesn't list twitter for the GB/GB combination, so if this is an issue, it's a configuration issue.

I've updated prod-preview to add twitter to en-GB. Adrian, please can QA test?

Tried on Ubuntu 18.04 En-GB localized build. + Windows10 64bit + MacOS 10.14

With Search extension helper addon:
Modernization turned ON:
Production active: twitter is missing
Production in review: Twitter is showing up

With remote settings devtools addon:
Prod preview with poll server and after the restart i was able to see twitter added on search engine.

This is now live on production.