How to configure proxy in web browser (Ubuntu, Mozilla Firefox)?

Open the menu in Mozilla Firefox, go to Edit and click Preferences.
Click the Advanced tab menu.
Click Network tab and then click the Settings... button.
Under Configure Proxies to Access the Internet, select the Manual proxy configuration option.
In the HTTP Proxy box, enter the IP address of Hostname of the proxy server.
In the Port box, type the port number which is being used by the proxy server for client connections.
Select the Use this proxy server for all protocols check box.
You can enter IP address that you wish to bypass the proxy connection in the No Proxy for box.
Click Ok to close the Connection Settings dialog box.
Click OK again to close the Preferences box.