This is a longstanding issue with Firefox's search bar, but the previous workaround, which was to enter "about:config" into the address bar and set browser.search.showOneOffButtons to false, became ineffectual in late 2015. Now that the active search engine is hidden, a web search requires the user to make two active queries: first she queries Firefox as to the active search engine; then she queries the desired search engine. Optimally, the active search engine is evident without query; the user need query only the search engine. Because searching the web is a core Firefox function, small optimizations and degenerations in search bar design are amplified in user experience.
Unfortunately it was decided not to implement any workarounds for this painful issue. It can probably solved by extension "Classic Theme Restorer". If you want to leave a comment - please do it in bug 1102922 (but it doesn't look very effective)
Thanks for pointing out the right place, arni2033. I have two or three cents to contribute there, for whatever good it does.