If you have not run ./verify.sh you can create the venv like this:
../common/setup-marionettesetup_firefox_ui_updates.sh venv
You can use --keep-venv and --developer-mode for this script. After you run verify.sh or setup-marionettesetup_firefox_ui_updates.sh, you can run the tests like this:
source venv/bin/activate (For Windows "Scripts" instead of "bin")