I don’t think there is one. And since it is planned to remove the search box from the default layout (but keep it as an UI option), I don’t think one will be added. So to answer your final question: No.
But Firefox will add the capability to add custom
iframe-based UI elements. You could implement your own search box with custom semantics.
I don’t understand why people like to type same content over and over again, every time they want to do the same search in a different engine.
Quite frankly, if you regularly need to consult more than one search engine, then your engines suck - or you don’t know how to use them. And to avoid typing stuff twice, there is the clipboard.