There is no other way to dismiss the modal, but the "X" button

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox Developer Edition 59.0b3
• Firefox Nightly 60.0a1 (01/26/2018)
• Min-Vid Shield Study Onboarding 0.4.7 - Shield Study

[Affected Platforms]:

• All Windows

[Prerequisites]:

• Latest Min-Vid v.0.4.7 Shield Study Onboarding ("activeAndOnboarding" variant) was previously installed on a clean profile.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Navigate to www.youtube.com website.
3. Press the "Esc" key and observe what happens next.

[Expected result]:

• The modal is dismissed.

[Actual result]:

• The modal remains displayed.

[Notes]:

• The other modals from the Test Pilot platform can be dismissed through different ways: "Esc" key, refresh, closing and reopening the tab etc. I really think we should add at least another way to dismiss the modal, besides the "X" button.
• Attached a screencast comparison between the issue and the modals from TP platform: