Why is there no Zotero icon in the Firefox Add-on Bar after installing or upgrading Zotero for Firefox?
First, make sure you have the latest version of Zotero installed by opening the Firefox Add-ons Manager via Tools → Add-ons. (On recent versions of Windows, the menu bar may not be shown by default, in which case you can go to the main Firefox menu and select Add-ons.) Zotero should be listed in the Extensions pane of the Add-ons Manager, and you should have version 4.0 or later.
If you don't see Zotero in the Extensions list or you don't have the latest version, install it from zotero.org. You should have the latest versions of both Zotero and Firefox.
If Zotero 4.0 or later appears in the extensions list, click on its Preferences or Options button to open the Zotero preferences and, in the General tab of the preferences, check that the Zotero icon is set to appear. Setting the icon to appear should automatically cause the add-on bar to be shown, but you can also manually enable the add-on bar in the Firefox View→Toolbars or Options menu. You should also be able to open the Zotero pane by selecting Zotero from the Tools menu (which may not be shown by default on recent versions of Windows but can be enabled via the Options menu).
If Zotero is listed in Add-ons → Extensions but doesn't show up in the Tools menu, you see the message “This add-on will be installed when Firefox is restarted” even after restarting, and/or you get other errors during installation, try the steps under Corrupt extension files on the Mozilla add-ons troubleshooting page.
You should also try disabling all other Firefox extensions, as some are known to cause problems with Zotero.