Unlike other web browsers, Firefox does not use your system's proxy Settings by default. You need to change your agent in Firefox:
1.Open the Firefox menu and select options.
2.Click the advanced icon.
3. Select the Network TAB.
4. In connection Settings, select the proxy configuration to automatically detect this network. When prompted, enter the proxy address.
5. Alternatively, in connection Settings, you can choose to use system agent Settings. If you select this option, Firefox will use the proxy Settings in your operating system.
6. Click OK to save your Settings.
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