If you're running Zotero 5.0 or Zotero Standalone 4.0, the word processor plugins are bundled with Zotero and should be installed automatically for each installed word processor when you first start Zotero.
You can reinstall the plugins later from the Cite → Word Processor Plugins pane of the Zotero preferences. If you're having trouble, see Manually Installing the Zotero Word Processor Plugin or Word Processor Plugin Troubleshooting.
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.
If you're running Zotero 4.0 for Firefox and want to install the word processor plugins, see Zotero 4.0 for Firefox Word Processor Plugin Installation.