One thing that has frustrated me lately is the challenge of adding new search engines to the the Firefox search window. Firefox options "Manage search engines" and "get more search engines" will bring you to this meager selection of search tools, but chances are your desired search engine - for example, an engine that searches the holdings of several hundred Scandinavian antiquarian booksellers at antikvariat.net - will not be among the listed addons. What you can do, however, is install "Add to search bar" - a wonderful little tool that enables you to add any search engine to the scroll-down selection in your Firefox search engine window.
Just place the cursor in the whatever search window you wish to add, and right click - the up pops the pop-up below
and hey presto, antikvariat.net's search engine is now among the selections in your Firefox search window