Firefox's Enhanced Tracking Protection and privacy-related add-ons may also block files in a way that sites think is caused by an "ad blocker." (A) Do you see a shield icon toward the left end of the address bar, near the lock icon? More info on managing the Tracking Protection feature in this article: [[Enhanced Tracking Protection in Firefox for desktop]] (before Firefox 70: [[Content Blocking]]). (B) Extensions such as Adblock Plus, Blur, Disconnect, Ghostery, NoScript, Privacy Badger, uBlock Origin, uMatrix and other content blockers should provide toolbar buttons 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 may need to click the button to see what's going on and test whether you need to make an exception for this site. Any luck?