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.