The version of Zotero that lived inside Firefox has been deprecated because the Firefox technologies it relied on are no longer supported by Mozilla. From this point forward, there is only one version of Zotero (Standalone), and all browsers (Firefox, Chrome, and Safari) connect to Zotero using a lightweight connector plugin.
@bwiernik: That's not the case yet, though. 5.0 isn't out yet, and even if nprager had installed the beta Zotero Connector for Firefox, it doesn't open Zotero itself.
@nprager: Assuming you're referring to Zotero 4.0, if you have Zotero Standalone open, clicking the Z button in the Firefox toolbar will just bring Zotero Standalone to the front. If you only want to use it within Firefox, you need to close Zotero Standalone. (But what bwiernik says will be the case after Zotero 5.0 is released.)
Upgrade Storage
More details here:
https://forums.zotero.org/discussion/comment/267549/#Comment_267549
@nprager: Assuming you're referring to Zotero 4.0, if you have Zotero Standalone open, clicking the Z button in the Firefox toolbar will just bring Zotero Standalone to the front. If you only want to use it within Firefox, you need to close Zotero Standalone. (But what bwiernik says will be the case after Zotero 5.0 is released.)