Zotero 5.0 and Firefox: Frequently Asked Questions | Zotero | Scoop.it
From www.zotero.org - November 5, 2017 8:29 AM

«…What’s changing?

Zotero 5.0 is available only as a standalone program, and Zotero 4.0 for Firefox is being replaced by a Zotero Connector for Firefox that allows you to save to Zotero as you browse the web, similar to the Chrome and Safari extensions that have been available for years for use with Zotero Standalone…».