Just had the same issue, but turns out you can just drag and drop:
"To install a plugin in Zotero, download its .xpi file to your computer. Then, in Zotero, click “Tools → Plugins” and drag the .xpi onto the Plugins window." https://www.zotero.org/support/plugins
"To install a plugin in Zotero, download its .xpi file to your computer. Then, in Zotero, click “Tools → Plugins” and drag the .xpi onto the Plugins window."
https://www.zotero.org/support/plugins