So many other image search apps in Firefox 57 have failed to work for me on Google Images when utilising its own search engine.
An image search attempt with all but one of those other apps that I tested resulted merely in a fresh, blank Google Images tab page opening for me to drag&drop an image onto........which is obviously not an action that I would need any extension app for.
And "Search By Image" (which is the only other image search app I discovered....besides your own....that is likewise working as desired with the Google Images search engine) has, since Firefox 57, stopped allowing the user to pick any individual image search engine to use (from a list of them) in any search attempt.........and instead searches via every single one of them simultaneously.......which is both resource hungry and very much overkill if that is neither needed or desired.
Yours is the only app that I've found that is currently working exactly as hoped with Firefox 57.....AND giving the user some measure of choice regarding how they conduct their search.
I also like the ability in the settings menu that you have provided to arrange the right-click, drop-down list of search engines into the order preferred.
So THANK YOU SO MUCH for this cracking app!!