Computer Configuration > Administrative Templates > Firefox > Search
Both
HKLM\Software\Policies\Mozilla\Firefox\SearchEngines\Add\5
If this policy is enabled, you can configure a search engine to be added to Firefox. Use {searchTerms} to indicate where the search term is placed. Due to a bug, you must select the method (usually GET). Note that you must specify the search engines in order. If this policy is disabled or not configured, a new search engine is not added.