In Firefox 29, Mozilla removed the Add-on Bar (formerly known as the status bar), where Zotero's icon has always been located by default. Zotero can now be opened from a new monochrome "Z" icon in the Firefox toolbar that matches the updated look of Firefox 29. If you only open Zotero infrequently and would prefer not to take up room in your toolbar, you can keep the icon in the new Firefox menu panel and use the “Open menu” button (three horizontal lines) in the Firefox toolbar to access it.

If you click the "Open menu" button and click Customize, you can drag the Zotero icon to the location of your choice or hide it completely.

As before, you can also use the keyboard shortcut (Cmd-Shift-Z or Ctrl-Shift-Z) or select Zotero from the Tools menu (hidden by default on recent versions of Windows).

When you first run Zotero in Firefox 29 or later, Zotero displays a popup pointing to the new toolbar icon.

I've also updated the support article with these details.