I tried syncing with Zotero standalone (no problems) and then went back to Firefox and syncing worked (without any changes to security software or proxy settings).
Earlier attempts at restarting Firefox and the computer also didn't work nor did deleting some of the extension files from the profile.
Probably security software, then. There's a good chance that future syncs in Zotero for Firefox won't work and that the only reason it seemed to work in Firefox was that there was nothing left to sync.
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Earlier attempts at restarting Firefox and the computer also didn't work nor did deleting some of the extension files from the profile.