first off, if you are concerned about security, privacy, etc. Update Firefox. The version you are on has got some significant security holes. Update Firefox to the latest release

Second, please give feedback at https://input.mozilla.org/feedback. Thanks!

Some sites code their pages so it is easy to add a search plugin. When the site comes back online, check the drop-down menu of the search bar to see whether it has an Add item similar to the one offered by DuckDuckGo (image attached).