In July 2017, Zotero version 5.0 was released! One of the biggest changes is that Zotero now runs only as a Standalone application. If you are running an older version of Zotero as a Firefox extension, you'll want to upgrade.
Read the announcement from zotero.org for details and instructions on how to upgrade!
Zotero is available for Mac, Windows, and Linux.
Image and text source: Zotero home page.