It seem that a lot of the tweet is missing, it doesn't have the usual styling, there's a reference to an image, etc.

If you right-click the embedded tweet, does Firefox has a "This Frame" item on the context menu? If so, and you expand that and click "Open Frame in New Tab" does the tweet render normally?

One reason embedded tweets might not appear is content blocking.

(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: What happened to Tracking Protection?.

(B) Extensions such as Adblock Plus, Blur, Disconnect, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix 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.

hello, are you sure you have allowed it to access media ? where are you using it ? i have two suggestions, i have faced this during my submission for college so it's a media that needs to load and somehow it went wrong,the second one is if you did give it the permission are you using it with another browser ? at the same device?

Not allowing Ghostery to block anything on the site has fixed it. Also, I don't have Ghostery added on to Chome or Opera thus explaining why there was no problem on those browsers.

The first thing I tried was turning off content blocking and that worked too, but I prefer to have that on.

Thanks for the help.