try this extension: https://addons.mozilla.org/en-US/firefox/addon/search_by_image/
after installing, go into the options and disable all search engines except for google. this will bring back the functionality i think you might be looking for

edit: no worries about the rating. the main purpose of this particular extension was to override the disable lens results -specifically- when accessed via 'search by image' on google's own website, rather than creating a context menu item (since there was already an extension that could do that). just wanted to clarify.