Just make a right click (Ctrl+click on a Mac) with the mouse pointer on top of *the page's* search box and choose "Add to Search Bar..." If you don't know how to use the extension, have a look at the second image - it's a screencast showing it in action.
![Add to Search Bar :: Add-ons for Firefox](https://cdn-ak-scissors.b.st-hatena.com/image/square/7bee9b3e3f7fbd861731e734ec4a6338a0f13db4/height=288;version=1;width=512/https%3A%2F%2Faddons.cdn.mozilla.net%2Fuser-media%2Fpreviews%2Ffull%2F10%2F10738.png%3Fmodified%3D1530208206)