How to add search engine using Firefox's address bar?

If I go to a website offering dictionary features, e.g. dictionary.com, I can easily add this search engine to Firefox using the search bar.

Say that I'm using Firefox without the search bar. How can I add a search engine using the address bar?

There's no option in the address bar for it as far as I can see; to add this search engine I changed preferences, added the search engine, then removed the search bar again.

Answer

There is now an option on the page action menu to add a search engine. This is available from Firefox 61 onwards.

From the release notes of Firefox 61.0:

Convenient access to more search engines: You can now add search engines to the address bar “Search with” tool from the page action menu when on a webpage that provides an OpenSearch plugin