See this post: https://support.mozilla.org/en-US/questions/1584995
When attempting to add a custom search engine, there is no guidance visible on the relevant menu as to how to format the link--and when Firefox detects no "%s" in the string the user supplies, it displays an error message that does not make sense in context.
Firefox clearly already checks for a "%s" in the string entered; to the extent practicable, I think Firefox should maintain a list of known additional search engine URLs and simply append the string itself. (Forcing users to read an article should be an absolute last resort, as 99% of users will absolutely not bother.)
