Complex sentence: In October 2009, Microsoft's security engineers acknowledged that Firefox was vulnerable since February of that year due to a.NET Framework 3.5 SP1 Windows Update that silently installed a buggy' Windows Presentation Foundation' plug-in into Firefox.
Split into simpler sentences: In October 2009, Microsoft's security engineers acknowledged that Firefox was vulnerable to a security issue found in the' Windows Presentation Foundation' browser plug-in since February of that year. A.NET Framework 3.5 SP1 Windows Update had silently installed the vulnerable plug-in into Firefox.