2.0.1 PPPoE Bug
There appears to be a bug in version 2.0.1 of PfSense. On the WAN interface, set for PPPoE, it does not allow you to save a "0" setting for "Idle timeout." You can save such a setting if you enter the value via the Setup Wizard, however, editing the WAN page appears to dump the setting. One can imagine what kind of problems this bug creates.
Erase the value so the field is blank. That should work and have the same effect.