Where Firefox Sends the Alerts from a PAC File

A PAC file can call alert() without opening a dialog. In Firefox, the PAC implementation writes those messages to the browser console with the prefix PAC-alert:.

Open Browser Console, rather than the Web Console attached to one page, and filter for that prefix. PAC execution belongs to the browser's networking machinery; it is not JavaScript running inside the website you are visiting.

alert("PAC host=" + host + " branch=office");

Place a short message in the branch you want to identify. If the script already computes a route variable, logging that value can show which result was selected. Keep the logging limited to the affected rule and remove it afterward.

Avoid adding dnsResolve(host) merely to make the message more interesting. It introduces a lookup that the original branch may not require, which can change latency and network activity while you are trying to observe them.

No message does not automatically mean the condition evaluated false. Confirm that Firefox loaded the intended PAC version and that the request caused a fresh evaluation. Diagnose loading and reuse before rewriting the rule around absent console output.