Man, I don't see this anywhere, so I figured I'd post it because I needed to created the true SEO forum URL, and not just a generic URL like fourm-1, use this global variable in forum display to print out the forum URL if needed for a link,
{$mybb->settings['bburl']}/{$sorturl}&sortby=lastpost&order=asc
{$mybb->settings['bburl']}/{$sorturl}&sortby=lastpost&order=asc