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

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 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 :)