Uses external service file.io
I expected this to work completely offline, but it uses external service, which does have its own Terms & Conditions and Privacy Policy, by the way. Just read the source code of the addon, and T&C with PP of file.io to see if you're OK with that before using this addon.

So cool!
I thank you in (almost) all languages of the world.
شكرا, շնորհակալություն, hvala, gràcies, 谢谢你, hvala, děkuji, takker, hartelijk bedankt, thanks, aitäh, kiitos, kiitti, merci, გმადლობ, გდმადლობთ, ευχαριστώ, धन्यवाद, köszi, takk, makasih, grazie, どうも, អរគុណ, 고마워, paldies, Благодарам, takk, تشکر, dzięki, obrigado, mersi , спасибо, хвала, vďaka, hvala, gracias, tack, ขอบคุณ, teşekkürler, Спасибі, cám ơn, …

:-)

Great. It coud be useful to have a contextual menu with right clic on the search box of the websites to add it more easely to the custom search engine. Have you thought to do it?

THIS IS PERFECT! This is the ONLY thing keeping me from switching to Firefox from Chrome, and I wasn't able to find it a week ago. Now, it is here, and it works just as well as Chrome. I can type my keyword, then hit the spacebar, and suddenly I'm searching in my Google Custom Search Engine.

FANTASTIC WORK, MR SCHUSTER!

GOOD

Seems very useful...
Just a problem (solved by last update: thanks):
With Waterfox , the addon icon is not visible (just an empty square)
In Firefox Quantum, that's fine.