Extensions with "@search.mozilla.org" in their ID are built-in search engines that can't be removed. You can hide them via "Settings -> Search", but they are still present. Some extensions are installed as built-in features (about:support => Firefox Features) and some can be disabled via about:config like for the screenshots (extensions.screenshots.disabled) and picture in picture (media.videocontrols.picture-in-picture*.enabled) if you do not want to use them, but you still see them listed. *https://support.mozilla.org/en-US/kb/about-config-editor-firefox