I'm guessing that these videos and images are embedded from another site, such as Twitter, Facebook, Instagram, etc. Sometimes Firefox's Enhanced Tracking Protection feature will block social media content embedded in other sites; users have definitely reported that for tweets. 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. See: [[Enhanced Tracking Protection in Firefox for desktop]]. In addition, content from Facebook and Instagram may be blocked by the Facebook Container extension (if you use that). I don't know whether you can make an exception, or you would need to use a link to view the content on FB/Insta instead.