[Affected versions]:
Nightly 67.0a1

[Affected platforms]:
Platforms: Windows 10 x 64, Mac OS X 10.14 and on Ubuntu 16.04 x64

[Steps to reproduce]:

1. Open Firefox and go to about:preferences, "Search" section.
2. In the "One-click Search Engines" check out the list of search engines.
3. Select one or more search engines from the list and click the "Remove" button.
4. Click on the "Restore Default Search Engines" button.

[Expected result]:
The previously deleted search engines are listed again.

[Actual result]:
"Restore Default Search Engines" button isn't restoring deleted search engines

Here is the regression range:
Last good revision: 9658187a54fbd9e7c5bda51fb4e5a041a7af77be
First bad revision: add76bbdce4e452d1f921bcbc1224b30892d8393
INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=9658187a54fbd9e7c5bda51fb4e5a041a7af77be&tochange=add76bbdce4e452d1f921bcbc1224b30892d8393

Looks as if Bug 1524593 landed in that range. ni on :mdeboer.

It seems that this issue was reported before.