Adding a new search engine via the URL bar

Open the "Page actions" menu

Click "Add Search Engine"

This option will not appear if a search engine with that name already exists.
Go to about:preferences#search to see your installed search engines.

Add custom search engine

%s is substituted with the entered search query

Icon should be at least 16×16 pixels - Data URLs are also accepted here

Tip: A keyword can be added by going to about:preferences#search after adding the search engine and editing the Keyword column

See the OpenSearch documentation on MDN for more information.

{searchTerms} is substituted with the entered search query (application/x-suggestions+json)

Due to a technical limitation with Firefox WebExtensions, all data entered when creating you custom search engine is uploaded to file.io. file.io stores this data temporarily. You can read their privacy policy and terms of service here: https://www.file.io/