This is most likely being caused by an add-on or extension, for example, the “Facebook Container” Extension made by Mozilla.
As of version 5.0 of the Pro plugin and version 2.0 of the Free plugin, you can configure the plugin to only use local images that aren’t sourced from Instagram/Facebook’s CDN. By default, the plugin saves a local version of each image in your feed. You can configure the plugin to use these images whenever they are available by going to the “Customize” tab, “Advanced” sub-tab or section, and enabling the setting “Favor Local images.” You’ll also want to make sure the image resizing feature is not disabled which is also found in this section.
