After the initial installation, the plugin will auto-update to newer releases, so you should need to perform the process described here only once. Install by downloading the latest XPI file. If you are not prompted with a Firefox installation dialog then double-click the downloaded xpi; Firefox ought to start and present you with the installation dialog.

For standalone Zotero, do the following:

  1. In the main menu go to Tools > Add-ons
  2. Select ‘Extensions’
  3. Click on the gear in the top-right corner and choose ‘Install Add-on From File…’
  4. Choose .xpi that you’ve just downloaded, click ‘Install’
  5. Restart Zotero