Firmware for the Slim Sloth can be found using the links below. There are versions for Windows and macOS which have different default keymaps to suit the OS — Windows (Ctrl/Alt defaults) / macOS (Cmd/Option defaults)

Original Stable Slim Sloth Firmware

Windows
macOS

Experimental Slim Sloth Firmware - VIA compatible. After flashing, visit https://my.grumpysloth.co/ to configure using Chrome or Microsoft Edge, Safari is not supported.

VIA - Windows
VIA - macOS

IMPORTANT NOTE: Flashing firmware or entering the bootloader via the instructions below will return settings to firmware defaults and remove any custom settings including any RGB lighting settings.

Firmware flashing instructions:

• Unplug your keyboard
  
• While pressing the Esc (Grumpy Sloth) key, plug the keyboard back in
  
• The keyboard will show up as a USB Disk Drive with the name RPI-RP2
  
• Copy and paste the firmware file to the RPI-RP2 drive
  
• The keyboard will restart with the new firmware installed
  

The Grumpy Sloth forks of the QMK firmware can be found using the links below

Stable/Old - GitHub
VIA - GitHub