https://blog.mozilla.org/en/mozilla/firefox-ecosia-partnership/ https://blog.ecosia.org/ecosia-firefox-switch/

Great news for Firefox users in Austria, Switzerland, Belgium, Sweden, Italy, the Netherlands, and Spain: Ecosia is now available as a search engine option in your browser! With France and the UK soon to follow.

There is an Ecosia search plugin in the source code (here) but I'm not sure which builds of Firefox get it automatically. I don't see it in U.S. English 64-bit Windows.

Until someone figures out how to surface it, you could use the OpenSearch installation method: while on the site, right-click the address bar and then choose it from the bottom of the context menu.