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.

checkboxes missing on my end for picking search engines. has this been encountered & replicated before?

Well, what can I say…: it works where others fail.

doesn't work

Not working with Google Image search.

[EDIT]
It works again! Great. I've changed star rating from 1 to 5 stars.

Google Image Search is broken

Doesn't work anymore :(

I usually roll CCleaner everyday browse because i browse many hundred of pages daily and FF cache doesn't work very well after such intensive browsing!
But I fixed by adding to exclude list on CCleaner
the following 2 files: https://imgur.com/a/k6kR800
Hope it will give you a clue since i am not a programmer like you.

On a Win 10 64bit, Firefox 88 (i need chrome folder for some tabs skin tweak)
This add-on "forgets" the 2 selected (and saved) search engines (Google & Yandex),
and searches just on Google (which is also set by me as the main search engine of the browser)
It needs a new save on the already selected engines to "refresh" the memory of the add-on.
Could you check what's happening?
Is there any temporary cookie or validity of the saved choice that will expire after awhile?

Thanks for this awesome add-on.

Her hangi bir veriye erişmiyor ve sizin için seçtiğiniz arama motorlarında istediğiniz görseli arama imkanı sağlıyor. Eğer farklı arama motoru eklemek isterseniz bunu ayarlardan yapabiliyorsunuz.