When you run "firefox", it will check if the current user already has an instance running; even if it's on a different display.

To prevent that you need to run the firefox command with "-new-instance" or "-no-remote" (which implies "-new-instance").

Firefox also 'locks' profiles which are in use, so you need to use separate profiles for each Firefox instance which is running. You can do that by making a temporary directory "T" and passing it to the firefox command as "-profile T".

As an example, the Firefox command I'm using in one of my scripts is currently:

    timeout 60 firefox -safe-mode         \
                       -profile "$FF_DIR" \
                       -no-remote         \
                       -new-instance      \
                       "$URL"

XFVB is an X server, which programs connect to based on the "DISPLAY" environment variable. You can see what that's set to by running:

    echo "$DISPLAY"

You can set it, e.g. to ":99", by doing:

    export DISPLAY=:99

Each running X server needs a different display number; this includes regular desktop sessions (usually running on Xorg with display :0), XVFB, Xnest, Xephyr, etc.

I use a variant of the script at http://stackoverflow.com/a/30336424/884682 to choose an unused display number for each XVFB invocation.

NOTE: This is based on simple automation tasks with bash and xdotool. I don't use Selenium, so I don't know if already does any of this. Hope it helps anyway :)