Find which rules trigger the FF23 mixed content blocker and mark them platform="mixedcontent"
Neither Firefox's Mixed Content Blocker nor HTTPS Everywhere will be ready for the other when Firefox 23 ships. On the Mozilla side, we need a fix for MCB firing too early in the request pipeline, and on the HTTPS Everywhere side we need to correctly label and disable those rulesets that cause problematic MCB situations.
For power users this isn't a problem, because they can live with broken CSS or manually allow mixed content. But for most of our users this is catastrophic.
So we will ship a patch which temporarily disables the MCB about:config setting if your Mozilla version is 23, but which pops a NotificationBox offering more information and letting you turn it back on if you'd like to.