Bugzilla

I think I've figured this out. STR:

1. Create a new profile and create a prefs.js file with the line `user_pref("browser.search.region", "RU");` (this presets the region).
2. Start up the Russian Firefox locale with Firefox 75. 
3. Wait 10 seconds, check the list in preferences and restart (this just makes sure that the profile is initialised etc).
4. Go to Preferences -> General -> Languages
5. Click the Set Alternatives button
6. Select "Select a language to add", and add English US
7. Select Add, then OK.
8. Select Apply and Restart.
9. Check the search engine list and the default engine.

Expected Results

=> "Yandex" should be the default.

Actual Results

=> "Google" is the default.

I suspect this would also happen if you started up a en-US build with the same profile rather than doing steps 4 to 8.

I think I've figured this out. STR:

1. Create a new profile and create a prefs.js file with the lines: (to preset region and turn modern config off).
```
user_pref("browser.search.region", "RU");
user_pref("browser.search.modernConfig", false);
```
2. Start up the Russian Firefox locale with Firefox 75. 
3. Wait 10 seconds, check the list in preferences and restart (this just makes sure that the profile is initialised etc).
4. Go to Preferences -> General -> Languages
5. Click the Set Alternatives button
6. Select "Select a language to add", and add English US
7. Select Add, then OK.
8. Select Apply and Restart.
9. Check the search engine list and the default engine.

Expected Results

=> "Yandex" should be the default.

Actual Results

=> "Google" is the default.

The same also happens if you started up a en-US build with the same profile rather than doing steps 4 to 8.

This also does not apply to the modern search configuration (only enabled on nightly at the moment).