Something to note about the new mixed content blocking:
it potentially breaks viewing certain sites with addons like HTTPS Everywhere:
They're working on a bugfix, but so far it remains unresolved afaik.
For those to lazy to follow the link:
Basically, even if HTTPS Everywhere redirects unsecure elements to their secure counterparts (e.g. makes example.com css loaded from http://example.com/example.css redirect to https://example.com/example.css), Firefox interprets the loaded page as having mixed content, and blocks the redirected elements, probably breaking the page design.