Do you mean making the search engines appear outside the "Search for (...)" sub menu? I tried to install Context Search on an older Firefox to see what it did exactly but it placed all engines inside a sub menu too, so I was a bit confused. :)
If it's that, I'm afraid I can't do it in WebExtensions (the only way to create extensions after Firefox 57). Sorry! Quoting Mozilla, "If you have created more than one context menu item or more than one tools menu item, then the items will be placed in a submenu." There doesn't seem to be a way around this.
