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Why don't I see a Zotero icon in the address bar when viewing a webpage?
If you've used Zotero for Firefox previously and no longer see a Zotero icon in the Firefox address bar, note that the location of the button has changed. See the 4.0.27 announcement for a full explanation of the changes.
If you're using Zotero Standalone and don't see an icon or Zotero for Firefox's toolbar button is only offering to save a webpage item, see Page Not Recognized.