The search box is separate in Firefox (next to the address bar). You can get to it by typing Ctrl+K (Cmd+K on Mac). Once you type your query, you can use the Down Arrow or Tab key to navigate to the site/engine you want to use and then hit Enter. You can also rearrange the order of those search engines to make the frequently used ones easier to reach.

If you don't mind a network round trip, you can also setup DuckDuckGo as your default search engine and use bang commands [1] to do different kinds of searches on many sites.

[1]: https://duckduckgo.com/bang