Bug 1799393 - Prevent hidden Fx View tab from triggering the closing multiple...

Bug 1799393 - Prevent hidden Fx View tab from triggering the closing multiple tabs warning in Windows and Linux r=Gijs

Updated the tab closing prompt logic to use the number of visible tabs open to determine if the prompt should show in Windows and Linux.

Differential Revision: https://phabricator.services.mozilla.com/D162627