I was always using 'pastego' for the old search bar:
https://addons.mozilla.org/en-US/firefox/addon/pastego/

I don't like the new search bar.
You need to make a right click > paste > select engine.

With the old old search bar and 'pastego' you also need to select the engine first.

If your Add-on would have a button we could click and then select the engine like we do in the context
menu and search for what is in the clipboard it would be perfect.

It also means there would be less work because adding all the engines only to your Add-on and no need to add them also to the Firefox 'one-click search engines'