At present, when a download link is an http link, Firefox blocks it because of the possibility of man-in-the-middle attacks, which is in principle sound. In most cases the http link is part of an old, no longer updated page, however usually it is hosted on a system which is updated which means that the https version of the download link will work.
These old http links will probably never be fixed and they're a major nuisance for Firefox users. Because usually the https links will work fine, let Firefox try the https version of the download first and only complain when that doesn't work either.
