On OS X, we center tab labels regardless of whether they overflow. When a tab label does overflow, the label's text is truncated at a discrete number of characters while the tabs are resizing, which results in the label jittering back and forth. This bug has become especially visible since we now have tab opening and closing animations. Unfortunately, the XUL text widget code doesn't expose to front-end code whether a label's text is overflowing/cropped, so we can't do something like the following, which would left-align a label when the ellipsis is present: tabLabel.onoverflow = function() label.setAttribute("overflow", "true"); tabLabel.onunderflow = function() label.removeAttribute("overflow"); .tab-label[overflow] { text-align: left; } A theoretical crop="fade" or text-overflow: fade; that behaved like Google Chrome's tab label overflow would also resolve this issue.

Not sure why you closed this as WONTFIX