Right-click in firefox and "save image". Drop it in /tmp so it gets nuked on next reboot (tmpfs yay). Pick your favorite barcode reader tool; I ended up deciding on `zbar-tools` based on... not honestly that much research. Invoke `zbarimg /tmp/qr.png`. It'll spit out the TOTP URI.

Verifying that I did it right: I manually enter the secret into a tool that can produce TOTP codes and verify that Google Authenticator and my backup are producing the same one-time passwords.