Mozilla Firefox

These instructions referes to the english version of Mozilla Firefox for Mac OS X.
With few changes they are probably correct also for other versions and different operating systems

Activate the "Preferences" or "Options" menu (it depends from the version).
Then click on the right on "Advanced" section to access to configuration section as shown in the picture:
Access to "Network settings" (or equivalent) as just shown in the previous picture.
Click on "Settings" on the right of "Configure how Firefox connects to the Internet".
Next you need to check the "Automatic proxy configuration URL" option and fill in the blanks the URL:
http://proxy.math.unipd.it/proxy.pac
After confirmation, clicking on "Ok" (or equivalent), configuration is done.