Rated 5 out of 5 stars

Thanks for your helpful response, the "Manage Engines..." item cannot work just because I use Omnibar extension to integrate location bar and search bar into one and hide the search bar, is it possible to solve it please?

https://addons.mozilla.org/en-US/firefox/addon/omnibar/

This review is for a previous version of the add-on (0.4.6.6).  This user has other reviews of this add-on.

I'm sorry, I think it's not the Omnibar add-on. I just installed it, and it still works. Perhaps it's another add-on.