[Enhancement] A message should be displayed, that the products can not be added to the "Tracked Products" list in Private Browsing

[Softvision-RemusDranca]

[Affected versions]:

• Firefox 63.0 and above

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest version of the "Firefox shopping" add-on (8.0.0) downloaded from (download link).

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and open a new Private Window (Ctrl + Shift +P).
2. Navigate to "https://www.amazon.com/" and open a product page (eg: product).
3. Click the "Tracked Products" toolbar button.
4. Click "Add This Product" button and observe the behavior.

[Expected result]:

• A notification should be displayed that products can not be added in Private Browsing.

[Actual result]:

• The button is not actionable and no notification is displayed.

[Notes]:

• Attached is a screen recording of the issue:
  

[muccimoz]

Requires new UX.