Consider the following purely hypothetical scenario: You do a lot of your work inside a web browser. Stuff on your workplace network (like the bug tracking system, the company Wiki and the source control system) you have to access directly. Stuff outside you have to access via a proxy (because the direct access is filtered in a haphazard way that blocks websites of your vendors and customers, as well as resources you need to do your job).

You can go manually change the network settings in your browser every time you go back and forth. You can set up two different browsers (one proxied and another not). You can use a browser add-on that lets you manually toggle proxy settings with one click.

Better still if you didn't have to do any of that stuff and your browser would just automatically do the Right Thing. With Firefox, you can achieve this result reasonably easily using a "Proxy Auto-Configuration File" (or PAC file, for short). Read on for details.
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