That message sounds like an address might have changed on the server...

Are you trying to access the site directly using a bookmarks, or by typing into the address bar, or by following a link in a search engine? Sometimes links change making bookmarks and search engine entries obsolete, or Firefox's autofill feature suggests an address that no longer works, so if one approach doesn't work, you might try one of the others.

One other thought... Firefox's autofill feature has a bias toward using a secure (HTTPS) connection on website home pages, even if history only shows that it has been used on part of a site (for example, at checkout). If the address that gives this error starts with HTTPS, try deleting the S and see whether that fixes it. (Maybe a secure home page is the service that isn't configured on the site.)