You can get your firmware from
4  different sources:
Choose the method most appropriate for your needs.
(On Windows Only)
This step is only applicable to a Windows environment, and only on the first time you run QMK_Toolbox.
Bootloader Mode allows your keyboard to be flashed with a new firmware. There are 3 ways to do this, choose one that works for you.
If done correctly the QMK Toolbox should pick up the MCU in bootloader mode and display its serial address in yellow text.
You can now start using the keyboard.