Well, if it is the same kernel running on both OS's, but a module isn't working in one of them, you might want to check the source of the OS. You may need to make adjustments to the camera relevant source code to allow it to interface properly with the kernel. Could also be that the kernel doesn't play well with FxOS. You could try flashing another kernel on the N4 and running FxOS on it after that to see if it makes any noticeable differences. Could be a few areas to check. Get with another developer or a kernel dev over in the N4 section and try to troubleshoot the issue to find where it is at.
Galaxy S3 - Unrooted Stock