this is a TECHNICAL PREVIEW FFS
its not even a beta release and might not be out for a year.
As for the browser debate, if you actually have to support end user PCs in a corporate environment then you pretty much need to be running IE. Firefox is a pain to deploy and an even bigger pain to manage, even doing things that are simple like setting it to use a proxy involves editing config files, etc. At least Chrome picks up most IE GPO settings so is easier to manage, no messing around with config files etc