Although technically the WebVR API is enabled on Linux, the back-end support for the API is not enabled by default yet, causing the API to never enumerate any VR devices.
The "Virtual Reality" permissions in the Privacy tab may be confusing to users, if it is not yet possible to use WebVR or WebXR sites in Linux.
There is much interested in enabling WebXR on Linux, especially as there is good support for SteamVR and devices such as the HTC Vive; however, It is not likely that this Linux specific work will be completed before the next merge from Nightly to Beta. The next merge will include the new permissions dialog options.
Perhaps we should consider hiding the "Virtual Reality" permissions options from the dialogue for Linux users until WebXR is enabled fully for Linux.