On Mac with Tabbed menu the tabs themselves seems aligned to the right and have a white block background. Others reported issue several versions ago and still not fixed.
Select User Interface as Tabbed or Tabbed Compact
Tabs rights alined on white background
User Profile Reset: No
Tabs should be left aligned and on continuous grey background
Created attachment 162209 [details]
Duplicate of bug 133214. If you disagree, fell free to change it back to UNCONFIRMED with a short reasoning.
*** This bug has been marked as a duplicate of bug 133214 ***