Google's search input is wrongly focused when accessing Voice Fill through toolbar button

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 57 and up
• Voice Fill v.1.3.5

[Affected Platforms]:

• All Windows
• All Linux
• All Mac

[Prerequisites]:

• Have a Firefox profile with the Voice Fill (v. 1.3.5) experiment installed.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Click the "Voice Fill" toolbar button and observe the Google search input.

[Expected result]:

• Google search input is not focused.

[Actual result]:

• Google search input is focused.

[Notes]:

• The user has to click on the overlay in order to focus it.
• Attached a screen recording of the issue: