[Linux] Min Vid player is no longer always on top if the focus is changed while a video/song is loading or playing

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox Nightly 60.0a1 (2018-01-25)
• Developer Edition 59.b3
• Min-Vid 0.4.7-dev (built on 01/25/2017)

[Affected Platforms]:

• All Linux

[Prerequisites]:

• Latest Min-Vid add-on 0.4.7-dev (built on 12/19/2017) was previously installed on a clean profile.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to any supported platform.
2. Send a video/song to the mini player and choose to play it.
3. Click on the Firefox window.
4. Observe the player.

[Expected result]:

• The player remains on top of the Firefox window.

[Actual result]:

• The player is hidden behind the Firefox window.

[Additional notes]:

• Here is a screen recording of the issue: https://goo.gl/KtBo1U