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
security.mixed_content.block_active_content
to 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.
EDIT: There’s a Plugin here to toggle this value, which might be safer then disabling it forever (there’s also one of toggling display_content). I’ll check them out.