Pertains to Firefox 23.0.1 and maybe later.
I was surprised this information was so hard to find, so hopefully this will make it easier for you.
At the company I work for we have an HTTP iframe inside an HTTPS web page. The new version of Firefox objects to this mixed content and forces the user to respond by clicking on a little shield icon in order to allow the mixed content. However Firefox does not "remember" this override, nor does it allow a "white list" of domains where you don't care about mixed content. You can however disable the checking entirely by changing these options shown in the graphic above. PLEASE do not do this unless you completely understand the security implications.
If this is what you were looking for, please leave a comment, just say Hi or something.