This article applies to the deprecated Zotero for Firefox (pre-Zotero 5.0) plugin. It no longer applies to the current versions of Zotero.
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).