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.

Works like a charm.

Does what it says on the tin. Easy to configure and no bugs that I immediately noticed.

Fantastic addon. Just adds a context menu and doesn't require ANY unnecessary permissions. Strong recommend.