We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
This message previously could appear when attempting to download Zotero Firefox extensions on systems or networks that intercepted secure (HTTPS) connections to zotero.org. We've since put a workaround in place that allows installations to take place over such connections.
Automatic updates will continue not to work over such connections, however. See Updates Not Detected for more details.