So far this is strictly anecdotal, but I suspect it is reproducible by others. Today I noticed that if ZAX's Program Control is set to "Max", and if there is a checkmark in the Custom setting for "Enable Timing Attack Prevention", this will cause the Private Browser tab of Forcefield Toolbar to become inoperable; i.e., private browsing can no longer be accessed. By removing the checkmark from "Enable Timing Attack Prevention", private browsing functionality is restored.
I'm mentioning this not because I need a solution/workaround (I already figured out the workaround, as above), but only as a reference in case someone else wonders why their private browsing has become inoperable. The presence of the checkmark affected private browsing in both Firefox and IE8.