Administrator style sheets have been updated in the 3.7.0 release to provided rounded corners on titlebars, tabs, and similar user interface elements.  Only browsers that support these CSS features - namely Firefox and Safari (or related gecko or webkit browsers) will enjoy these changes.  Users of out-dated browsers (including Internet Explorer) will continue to see regular square corners.

You may need to hard-refresh to force the new stylesheets to load following an upgrade.