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?