On Windows, Safari uses IE's settings for its proxy support. In the preferences dialog, clicking the [Change Settings..] button for Proxies opens the Internet Options dialog (Connection Tab). Apple seem to consider it a system-level control panel, apparently. Felipe is working on a new IE migrator, so I'll hold on with my copy-paste for now (or cut & paste to MigrationUtils.jsm. We'll see).

Hrm, I just noticed bug 485764, and it is turned on by default. Oddly enough, the current IE migrator still imports the settings from IE. Felipe: make sure not to copy that. :)