Based on what I'm seeing here, there is something in your browser configuration that prevents Twitter's iframe from being loaded. I notice that there should be a small Twitter logo in the top right corner of the page that says "Loading tweet" and if that image is missing it's possible that the same thing that blocks iframes also blocks images, and subresources in general. Try a new Firefox profile or disable all of your Firefox extensions and you may find what it is. Otherwise, look into your browser console for error messages.