NOTE Since Firefox 140, there is native support for adding a search engine via the settings.

Add a custom search engine to the list of available search engines in the search bar and URL bar.

Your custom search engine needs a name and search URL. Additionally fields include an icon, a suggestion URL, POST query parameters, the input encoding and a description.

Icons can be selected from your local computer or specified as an URL pointing to an image.

The extension UI is accessible via a button right next to the URL/search bar or the options in about:addons.

Due to a technical limitation with Firefox WebExtensions all data entered when creating your custom search engine is uploaded to a third-party service.