Click on the Zotero icon at the bottom of your Mozilla Firefox browser window to open the pane. You can add a Zotero icon to your toolbar at the top of your browser window by clicking on View → Toolbars → Customize. You can close the pane by clicking the Zotero icon (either in toolbar or on bottom of your browser window or by clicking on the X icon on the right. You can also open or close pane from the keyboard: Ctrl-Shift-Z on Windows/Linux or Shift-Command-Z on Mac OS X).