VDT doesn't change the default behaviour of third party software. You have to look in software documentation for command line options or for registry settings.

For example, Internet Explorer starts all its windows within a single process; to start IE in a separate process, you have to use the -New command line option.

I don't know if Firefox supports such an option.

Another solution is to create a non virtual desktop (instead of a virtual one). Doing so, programs running in the non virtual desktop will not "see" the others that a running in other desktops; Firefox should start in this non virtual desktop. However, some program really don't support multiple instances (but this is not a VDT issue), so you will have to test if all the Firefox instances are working properly.