Adding Firefox 24,27 and 28 for SCL3 Windows nodes

I've installed Firefox versions 24, 27 and 28 on our SCL3 nodes. This win7.json file will completely replace the win7.json we use on our VMWare instances.  I'll take down all the VMWare instances after this is merged and configure all our Windows nodes to run in SCL3
Why increasing maxInstances? We still won't be able to run instances in parallel.