TOTP Auth от Mr. Smirk

This extension manages an OTP for you. If activated, you can click it to quickly generate a verification code and copy it to your clipboard.

If you have an activation QR-Code; if you already have a URL, skip step 1:

1. Extract the URL from the QR-Code (for example using https://scanqr.org/)
2. Install this extension
3. Click on the icon in the top right corner of the Firefox window.
4. Enter the URL that you got from the QR code. Click "Save".
5. Click the extension when you need an OTP code. It will copy the code to your clipboard.
6. Ctrl+V to paste it into any form field :)

Disclaimer: Only supports SHA-256 as algorithm.