If you've used Zotero for Firefox previously and no longer see a Zotero icon in the Firefox address bar, note that the button has moved to the Firefox toolbar. See the 4.0.27 announcement for a full explanation of the changes.

If the Zotero for Firefox toolbar button is only offering to save a webpage item, or if you're using Zotero Standalone and don't see an icon, see Page Not Recognized.