TypeError: Zotero is undefined - Firefox

Whenever I start Firefox, a message shows up:
"TypeError: Zotero is undefined". This is even if I am not trying to use Zotero, but just Firefox.
Also, the little Z that is supposed to be in the Firefox bar is not there any more, so I cannot add stuff that I find online to my Zotero library.
Does anyone know how can I solve this?
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