Since the tabs size and padding has been updated, the max number of tabs per row is no longer 6. There is also a plan to make the number of tabs per row dynamic based on an adjustable sidebar main width. Additionally, There is a proposed patch to pack pinned tabs when expanded - Bug 1921081. Hence, this patch aims to make the `maxTabsPerRow` variable dynamic too. Differential Revision: https://phabricator.services.mozilla.com/D224315