You will have to go to the Addon site and find the Addon you installed to see what is missing that it doesn't load or work.

The support article currently shows images of themes. Try restarting Firefox in troubleshoot mode to see whether that makes a difference. Your current theme may be the cause.

https://support.mozilla.org/en-US/kb/use-themes-change-look-of-firefox

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems https://support.mozilla.org/en-US/kb/troubleshoot-issues-related-to-add-ons

The theme images on about:addons are online and cached locally. If you cleared the cache and there is no internet access or a connection cannot be established with the server, then the images will fail to appear.