Fantastic. Open source, no permissions needed, can search multiple providers (Yandex is actually pretty good!).

It was a little hard for me to find the settings. You have to go to Firefox' extensions settings, and on the Reverse Image Search details page (you can also get here directly from the extension's own icon, via the "Manage Extension" link), you click the three dots icon to the right of the on/off toggle button and there's a link to the the preferences.