Zotero senses information on web pages through site translators, and a Zotero icon will appear only if a site translator recognizes the page being viewed. Zotero's translators should work with most library catalogs, popular websites such as Amazon and the New York Times, and many gated databases.
If a site isn't currently supported or a translator isn't working, you can still save any web document, though you may need to fill in some details that Zotero couldn't automatically detect.
If you don't see a Zotero icon on Amazon product pages or NY Times articles (not the home pages), see Troubleshooting Translator Issues.
If you see the icon on Amazon product pages and NY Times articles but don't see it on another supported site, you can report it in the forums. Be sure to provide an example URL.