Edit: saved configuration, reloaded default then loaded the saved one and everything started to work. I have vanilla Firefox.
5.9 does not work on Firefox 57. It does nothing when clicking on a search engine. Seems when something gets fixed something else break.
Have the following installed: Awesome RSS;Bookmarked Speed Dial;Bookmarks Organizer;Close Tab History Button;Cookie AutoDelete;Decentraleyes;Default Bookmark Folder;Download Star;Evernote Web Clipper;Form History Control (II);HTTPS Everywhere;Image Search for Google;In My Pocket;Kee;Linkificator;MyJDownloader Browser Extension;Pushbullet;Reader;Reading List;Selection Context Search;small-clock;Snap Links;Tampermonkey;uBlock Origin;Update Scanner;Video DownloadHelper;Viewhance;Tweetdeck sidebar
Edit: I meant to ask if you use some variations of Firefox like Waterfox, Pale Moon, ... Or if the Selection Context Search was installed other than from mozilla.org. I do not know why it is not working for you. Maybe other add-on is interfering? You can save the settings, uninstall the add-on, install it again and load the old settings. I will look more in the code for a possible cause. Please let me know if you suspect a cause.
Do you happen to have something else than a regular Firefox with a standard add-on installation?