Have to create a bookmark folder first, and then restart FF, really? Better use the web-extension called "contextual-search". https://cup.aihedy.com/en-US/firefox/addon/contextual-search/reviews/
The restart requirement is worked on (there's an open issue on GitHub), but since most people rarely add new search engines to the list it has been a low priority.
EDIT: The restart requirement is now removed in version 0.9.5, available as we speak! :)
