Zotero Addon for firefox being disabled
The new update of firefox is disabling Addons that do not have signatures. Zotero, apparently, does not have a signature and is being disabled. A work around is to turn off the need for signatures in about:config. However, the better solution would be for Zotero to create a signed version of the addon for firefox
If Firefox is disabling Zotero, it's highly unlikely you have the latest version.
Next to the disabled notification there was a link to check for updates. I clicked that, an update to Zotero was found, and I was able to let Firefox install the update for me. After restart, I have Zotero back.