• Multi-Account Containers Version: 7.0.2
• Operating System + Version: Windows 10 x64, 1909
• Firefox Version: 78.0.2
• Other installed Add-ons + Version + Enabled/Disabled-Status:
  Bookmark Tab Here 1.0 true {bc21b9a1-3ad6-4b00-bca4-ef5b7e21253b}
  Copy Current Page URL with Anchor 1.0 true jid1-grAPHQfoBoWMdA@jetpack
  Copy Link Text 1.6.1 true {b144be59-6bdc-41e0-9141-9f8d00373d93}
  Copy PlainText 1.11 true copyplaintext@eros.man
  Download Manager (S3) 5.12 true s3download@statusbar
  Enhancer for YouTube™ 2.0.101 true enhancerforyoutube@maximerf.addons.mozilla.org
  Facebook Container 2.1.1 true @contain-facebook
  Firefox Multi-Account Containers 7.0.2 true @testpilot-containers
  Gesturefy 3.0.5 true {506e023c-7f2b-40a3-8066-bc5deb40aebe}
  Refined GitHub 20.7.16 true {a4c4eda4-fb84-4a84-b4a1-f7c1cbf2a1ad}
  RoboForm Password Manager 9.0.1.6 true rf-firefox@siber.com
  Stylus 1.5.11 true {7a7a4a92-a2a0-41d1-9fd7-1e92480d612d}
  uBlock Origin 1.28.2 true uBlock0@raymondhill.net
  uMatrix 1.4.0 true uMatrix@raymondhill.net
  LastPass: Free Password Manager 4.51.0.1 false support@lastpass.com
  To Google Translate 4.0.6 false jid1-93WyvpgvxzGATw@jetpack
  Worldwide Radio 4.1 false worldwide@radio
  Xdebug-ext 1.2 false {5483b263-bf97-4858-a2cf-590444554869}

Actual behavior

There is no black colour when editing/viewing container settings via Multi-Account Containers popup panel. But it is there when managing container via Firefox preferences.

As a result the container will loose black colour and it will be reset to default/1st in the list (which is blue).

Expected behavior

The black colour is present and remain assigned.

Steps to reproduce

1. Multi-Account Containers popup panel
   
2. Click on "Manage Containers" at the bottom
3. Locate desired container (e.g. Facebook) that has assigned black color to it
   
4. Click on it to bring the editing panel page (e.g. you want to see what domains are assigned to this container)
   
5. Do not edit anything (or edit, it does not matter) and click "<" button to go back
   Expected: black colour remains assigned
   Actual: black is gone, it is now a blue colour instead.
   

To fix it I have to change the colour via Firefox Options: about:preferences#containers