When you set any one search engine as default in Firefox from the context menu then Mozilla’s browser only allows to search for selected query with that search engine only, what if you want to search selected text on page with other search engines like Bing, Yahoo ,Wikipedia came  installed with Firefox, Context Search Firefox add-on expands the context menu’s ‘search for item’ into list of installed search engines, and allows you to choose search engine you want to search.

search for item in Firefox Context menuContext Search Firefox

Context Search is more useful as you don’t need  to  switch the search engines from one to other for the selected query to search from the context menu.