What is this script for? If you make FireFox open the application in a new tab instead of a new browser window, one negative side effect is that you are unable to get the focus back into the builder, as Firefox doesn't support that. The only workaround - slight annoying - is to show an alert in the builder window, which will force the browser to get focus back into the tab that shows the alert message.
Install Greasemonkey or Violentmonkey, and install the script.
Trent Schafer
MIT