Impressive and very complete extension.
It allows me to synchronize all my global search engines using bookmarks. I also add custom local search engine depending on my need.
I don't know why this is not an extension with the Mozilla's "Featured Extension" badge (for instance "Swift Selection Search" has the badge, but is really less powerful).
One issue though:
When using bookmarks, the separators are taking the same height as a normal bookmark in the context search menu. Couldn't you display it like a object in the local Selection Context Search menu (reduced height)? I will review my rating to 5 stars is this issue is fixed.