Unable to open the "Learn more about web compatibility" modal using the keyboard

[cipriansv]

Browser / Version: Firefox Nightly 81.0a1 (2020-09-17) Operating System: macOS 10.15.6

Steps to Reproduce:

1. Navigate to https://webcompat.com/issues/new
2. Using "Tab" key, navigate to the URL field and fill it in.
3. Using "Tab" key, navigate to "Confirm" button.
4. Using "Tab" key, navigate to "Learn more about web compatibility" link.
5. Using "Enter" key, activate the modal.
6. Observe behavior.

Expected Behavior: The modal is displayed.

Actual Behavior: The modal is not displayed.

Notes:

1. Screenshots attached.
2. The issue is not reproducible on Chrome 85.0.4183.102
3. The issue is not reproducible on Windows 10 Pro.