Virtual memory is allocated per-process. If you think it's connected to other processes, then it may be a problem of the non-paged memory usage. It's used by drivers to allocate memory for communication with the GPU and other devices. Non-paged memory is a limited resource, and more apps using it can cause problems.
You can try to turn off GPU acceleration in Firefox. Also, with recent versions of Firefox (22-23) and with a Nvidia card I have experienced the Windows freezing hard several times, requiring reset, so I think there's definitely a problem with the GPU acceleration in there.