Currently, when the tab strip is in focus, pressing the arrow keys currently switches the active tab only. With this change, pressing the arrow keys will continue to switch the active tab when the adjacent item in the tab strip is a tab, but tab group labels will also receive keyboard focus if they happen to the adjacent items in the tab strip. Now, pressing left/right allows the user to "select" each tab or tab group label in the tab strip.
Differential Revision: https://phabricator.services.mozilla.com/D233016