I am a loyal Firefox user and recently started trying out the vertical tabs feature. However, I noticed a significant UI issue: there is a very sharp right angle in the upper-left corner of the web page. Currently, Firefox employs a rounded-corner UI design overall, and this sharp angle in the upper-left corner looks quite inconsistent, especially since the tab to its left and the address bar above it both feature rounded corners. Therefore, I kindly request that this issue be addressed in a future update to provide a better visual experience when using vertical tabs.