The word processor plugins are bundled with Zotero and should be installed automatically for each supported word processor on your computer when you first start Zotero.

You can reinstall the plugins later from the Cite → Word Processor Plugins section of the Zotero settings. If you're having trouble, see the troubleshooting instructions.

If you previously installed the Firefox versions of the word processor plugins into Zotero 5.0 or Zotero Standalone 4.0, you should uninstall them from Tools → Add-ons.