For example, the recently released Firefox 23 blocks any active content requested via http from an https page.
For XenForo, I assume if users embed a YouTube video, the Iframe src will be http, and the content would be blocked.
I can't test that in the test forum here, because XenForo.com redirects https requests back to http.
And I don't have a real certificate yet to test on my site.
Is anyone running all-SSL at the moment? What kinds of issues have you had?