Firefox becomes unresponsive after closing all tabs with keyboard shortcuts

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 53.0 and up
• Tab Center v1.34.0 (release version)
• Tab Center v1.34.0 custom build

[Affected Platforms]:

• All Windows
• All Linux
• All Mac OS

[Prerequisites]:

• Tab Center was already installed on a new profile.
• Have at least 3 opened tabs.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Press and hold Ctrl+W (Cmd+W) for few seconds (~3 seconds).
3. Observe the way the tabs and the browser window are closed.

[Expected result]:

• All tabs and the browser window are closed.

[Actual result]:

• Firefox becomes unresponsive and the tabs are wrongly overlapped in the Sidebar.

[Notes]:

• The issue is not reproducible on a regular build, without having Tab Center add-on enabled.
• After few seconds, a "Warning: Unresponsive script" error is displayed: https://goo.gl/DeU2Po
• Unlike Closing a large number of tabs & windows with tab center enabled hangs the browser #1053 report, I must note that this behaviour happens on a clean profile, with no add-ons installed beside Tab Center.
• This also happens using Ctrl+F4 and middle click shortcuts.
• This is not reproducible with only one or two opened tabs.
• Here is a screen recording of the issue: