Someone pointed out to me that you can set eBay as the default search engine. That seems like a weird option. There can't be many that primarily uses their browser to access eBay, though I wouldn't rule out that people with this preference exists.
Stock Firefox also allows setting Amazon and eBay as a default search engine, and I wonder if this is related to Amazon being a sponsored shortcut on the new tab page.
I wish you could just set the default search engine to "none". I just disable it eventually anyway, but it still leaves a bunch of garbage behind, like having "Search <whatever> for <blah blah blah>" in the context menu.