I check the pop-up blocker box in Options, but after a while I find it unchecked -- usually after I reopen the browser.

Firefox help said McAfee can change preferences, but when I clicked on the link it provided in response to that problem (the link said ``Pop-up Blocker''), it didn't discuss the anti-virus issue. I don't know if McAfee is the problem, though. And the problem of finding the pop-up box unchecked has persisted through several Firefox software updates.