A couple of weeks ago, I suddenly couldn't see Tweets embedded anywhere outside of Twitter. I can't see Instagram embeds or Facebook embeds.
I can't even see Facebook comments on sites that use them, like Buzzfeed. Come to think about it, I can't even see Buzzfeed's in-house comment system anymore. Everything looks fine when I go to Twitter or Facebook, just not on other sites. I still post embeds at more than one site, and no one has complained about not being able to see them. I just get a blank box for Instagram and plain text for Twitter embeds.
I Googled the question, and the results make it appear that no one has ever had this problem before. I use Firefox on a 2016 iMac running High Sierra. I've rebooted, repaired the disk, installed all updates. I do not use AdBlock. Anything else I can try?