The "Password:" string is no longer displayed after adding a password

[Softvision-CristinaBadescu]

[Affected versions]:

• Firefox 56 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Steps to reproduce]:

1. Open the browser and navigate to https://dev.send.mozaws.net.
2. Upload a file and select "Require a password to download this file" checkbox.
3. Type a password and click the "Add password" button.
4. Observe the password preview.

[Expected result]:

• The following text appears: "Password: [the password you typed]".

[Actual result]:

• The "Password:" string is missing and only the typed password is displayed.

[Notes]:

• Here is a screen recording of the issue:
  

[Softvision-CristinaBadescu]

I can confirm that this issue is no longer reproducible using latest Nightly build 58.0a1 (2017-11-06) on Windows 10 x64, Mac 10.12 and Ubuntu 14.04 x64.