I think the problem is solved but this is what was happening: Whenever I restarted Firefox the default search engine was set to Yahoo. Since version 53 of Firefox Yahoo hasn't been one of the search engines in Firefox. I haven't, knowingly, loaded Yahoo and certainly wouldn't have done so given the option. Even new builds of Firefox retained Yahoo as the default search engine, only a refresh of Firefox seems to have banished Yahoo. I am assuming that something I downloaded came with Yahoo attached and I either didn't notice or wasn't given, the option not to load Yahoo. Anybody have any ideas what I might have downloaded that carried this unwanted passenger? Is there a way to remove it, other than a refresh?