From MozillaZine Knowledge Base
Mozilla provides a website with downloadable search plugins for its Search Bar. A link is provided in the search engine dropdown. This preference determines where the link goes.
Possible values and their effects
The URL to visit when a user selects “Add Engines” from the Search Bar dropdown. (Default: https://%LOCALE%.add-ons.mozilla.com/%LOCALE%/firefox/%VERSION%/search-engines/)
First checked in
Has an effect in