The Zotero Connector allows you to save items from your web browser into the Zotero desktop application on your computer. (It can also save some pages directly to zotero.org, but saving to the Zotero application provides the best experience.)
First, make sure that Zotero is installed and open on your computer. If you don't yet have the Zotero application, you can install it from the downloads page. If you were previously using Zotero for Firefox, note that Zotero now runs only as a standalone application.
If Zotero is open but the Zotero Connector reports that Zotero is unavailable, something on your computer is preventing the Connector from talking to Zotero. You can determine whether the problem is within the browser or a system-wide problem by loading
http://127.0.0.1:23119/connector/ping in one or more browsers while Zotero is open. Under normal circumstances, it should display “Zotero Connector Server is Available” (or “Endpoint does not support method” in older versions).
23119in the extension's settings.
127.0.0.1(an alias for your computer itself) is excluded from proxying, and then check
http://127.0.0.1:23119/connector/pingstarts working, reenable each piece of software one at a time and, if the problem recurs, whitelist
23119in the software's settings.