Welcome to the community forums and thanks for making us aware of a problem!
Firefox unfortunately really badly implemented that feature where extensions can define their own activation and search patterns.
Unlike in Chrome, the search results do not show the memex icon, but that green puzzle piece. It also does not switch the indicator in the search bar to this, but just keeps showing the "m " in the address bar.
Essentially you just need to continue typing and it will show you the results like this:
Hope that helped!