This can be caused by Firefox's Tracking Protection feature, or by other blocker add-ons: (A) The shield icon toward the left end of the address bar usually turns a bit purplish when content is blocked. Click the icon to learn more or make an exception by clicking the blue slider switch to the off position. See: [[Enhanced Tracking Protection in Firefox for desktop]]. (B) Aside from ad blockers, extensions such as Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, and uBlock Origin may block required elements on a site. Usually you can find a toolbar button for each add-on to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you sometimes need to click the button to see what's going on and test whether you need to make an exception for this site. Any progress so far?