Being an owner of Lightroom 5, I was asked to upgrade my installation to Lightroom 5.6 . After downloading the installer asks for a location to unpack Lightroom before it installs, and at that point it throws an error at you stating there was a problem while installing Lightroom 5.6 and it kindly asks you to download the product again.
When you do download it again, of course it fails over and over. I could not get to the root of this issue, until I figured I’d start the installation from disk directly an not through my Firefox broswer. Duuh! I don’t know what Adobe has done to this installer to make it fail, but running the installation directly from the download location through the Windows Explorer works perfectly.