Well... Everything that you'd use a regular VR app for, I guess. It's not quite clear yet what the most important use cases for VR will be, but Mozilla appears

Well... Everything that you'd use a regular VR app for, I guess. It's not quite clear yet what the most important use cases for VR will be, but Mozilla appears to be betting big on providing all API's that might be required to make the web just as capable of providing VR experiences as regular apps (i.e. what ideally would have happened with web apps in the first place), and for that, they need a VR browser. In other words, you'd be able to play a VR game without needing to install anything, by simply visiting a Web VR app, and that app would work on all VR platforms that run VR browsers like Firefox Reality.