Firefox has a feature that prevents loading insecure content in secure (https) connections. Unfortunately this makes some websites almost unusable because you have to disable this per page not per domain, so if you load more then one page on a domain you might do it again and again and again (srsly, wtf).
Luckily this is Firefox and you can disable this in
about:config by searching for
mixed_content and setting the value of
false by double-clicking it. (Via)
Note that I’m pretty sure that doing this is a SECURITY RISK because I don’t think Mozilla has this feature for the lolz. I’ll live with that though, until they’ll change this to blocking-per-domain.