All the URLs native to WXForum.net automatically use HTTPS for access if the forum is accessed with https://
The parts that return 'mixed' are based on member supplied URLs for Avatars and Signature images, and not all the member sites have HTTPS enabled, so I'm loathe to force a rewrite of *all* URLs supplied by HTML in the forum pages. I've updated my Signature to use HTTPS for my banner image, and hope that others who have HTTPS available, will do the same .. forcing the conversion would just 'break' a bunch of the images (all offsite [img]http://...[/img] imbeds, for example).