Could it be that this is an innocent mistake or a bug in the way that Google servers are sending HTTP. Chrome wouldn't be affected as it will be using SPDY for all Google services. As soon as Chrome switched back to the Firefox user agent it started using HTTP again and the same bug was found.
Besides, this just doesn't make sense. If this was an attempt to make Firefox look bad then it's a dreadful one. This just serves to make Google services look faulty as Firefox will still work for everything else. Because of this I doubt there is any malice behind this and it is just a bug.