Copy Paste in Firefox results in function getSelection() { [native code] }

🐛 bug report

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project
  adheres to.
• I have searched the issue tracker for an issue that matches the one I want
  to file, without success.

Description of the problem

When I copy code in the beta editor my next paste does not contain my copied code, but rather

function getSelection() {
    [native code]
}

How has this issue affected you? What are you trying to accomplish?

Trying to copy code in beta sandbox and paste it elsewhere in the sandbox or outside of it

To Reproduce

1. Open any beta sandbox in Firefox
2. Copy code
3. Paste code anywhere
4. See bad result

Link to sandbox: link (optional)

Your Environment

Doesn't seem to be an issue in Chrome

Start by reproducing the copy-and-paste steps in the beta editor using the linked simple-area-chart sandbox, first in Firefox and then in Chrome for comparison. The work is done when copied code pastes as the original code in Firefox, both inside and outside the sandbox.