Firefox IOS uses the safari browser because Apple's walled garden is off limits for browsers that are not based on that. So, that makes shipping a VR browser without the Firefox internals impossible/impractical. In this case, it's not only a browser but a content store that is not iTunes, which complicates things further.
I'm not aware of any iOS-based VR headsets.
They are probably not targeting the el-cheapo cardboard variety.