After months of hemming and hawing on plans for a standalone Internet Explorer upgrade before Longhorn, Microsoft Corp. now plans to push out a browser refresh by July or August this year. But the news that IE 7.0 will be available only to Windows XP SP2 (Service Pack 2) customers isn’t likely to sit well with security experts who argue that the threat from the Firefox browser is at the center of Microsoft’s aggressive anti-spyware and anti-virus plans.