dstiles - 8:04 pm on Apr 16, 2013 (gmt 0)
It has long been the case that the more responsible browsers (including firefox) warned about mixed content. It has never been good to serve (eg) non-SSL images with an SSL page. I've had that warning enabled for years in firefox and only occasionally accept mixed content if I know the site well.
In any case, if a site is properly designed it should never serve mixed content.
It's good that FF now defaults to that mode but I wonder if they force it upon existing users after an update.