FeFoo is a handy multi-search engine with a neat and orderly interface. There is an option to add Fefoo to the list of search engines in the Firefox dropdown search, but I couldn't get it to work...nor can I imagine how it's supposed to work, since fefoo is not a search engine, but (like the Firefox search window itself) merely an interface that coordinates access to many search engines. When installed among the search engines in the Firefox dropdown, fefoo simply defaults to Google...and sticks to Google, even if you go to fefoo and select a different engine. The best way to use it, as far as I can see, is to pull it up via a bookmark.
p.s. - re my remarks about Firefox addon above, see helpful comment by Vivek Jishtu about using commands in the Firefox dropdown window to specify search engines, e.g. ":bing libraries" will search bing for "libraries."