This window is opened by double clicking a script which you can modify - or by right clicking it and selecting Open. It edits, compiles, and saves scripts written in LSL. For more information about scripting, visit the LSL Portal.
If a script is no-modify, you are notified that “You can not view or edit this script…”.
See this page for documentation on Firestorm 4.7.1 (45325) and earlier.