Notice for Firefox 78+: Due to changes made by Mozilla, the add-on is now less convenient. I apologize for the inconvenience.
You can add a custom search engine to the list of available search engines in the search bar and URL bar.
Your custom search engine requires a name and search URL. Additional fields include an icon, a suggestion URL, POST query parameters, input encoding, and a description.
Icons can be selected from your local computer or specified as a URL pointing to an image.
The extension UI is accessible via a button next to the URL/search bar or through 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 paste.mozilla.org.
