Since tabs are listed in a grid, there are two ways to visually show "Close" affordance: Hidden behind thumbnail and triggered by swipe OR displayed on top as an overlay. As Fennec has built swiping up to close a tab[a more efficient way to close tabs], overlay on top is the best approach in this case. The hit area needs to be precisely measured and clear affordance is important, in order to avoid tapping mistakes. [This is one the three ways to close a tab from tab panel. The other two interactions are swiping up gesture, and long-press to enter reordering + deleting state.] Next step: * Visual design * Animation refinement * Implementation

This is already implemented (bug 1076898). Still needs general UI polish (bug 1064415)