If you are unable to load Web pages in Safari, connect to the iTunes Store in iTunes, and otherwise unable to access network services through Apple applications (and some third-party applications), but are able to access network services through applications like Firefox, the problem may be an invalid proxy setting.

Apple's applications and many third-party apps make use of Mac OS X's proxy settings, which can be configured in the Network pane of System Preferences. If these settings are invalid or incorrect, you will likely have problems accessing the Internet through apps that make use of them.

Navigate to System Preferences, click the Network pane, then select your currently used interface and click "Configure", then select Proxies. Make sure that your proxy settings are correct, or delete any entries if you are not using a proxy.

Some third-party applications don't use the built-in Mac OS X proxy scheme. A notable member of this category is Firefox. This is why you'll still be able to access the Internet through these applications when your Apple apps and others that make use of the proxy settings are disabled.