Mouse gesture lookup and "ugly tab"

[Pox]

When I use the mouse gesture lookup it opens a new tab which is
visually different from the others. It's few pixels taller and seems
to have an ugly frame on top. But the worst part is, that it moves
everything down few pixels. It looks like Firefox tries to align the
other tabs with the new one and subsequently moves everything down a
bit. It's not very pleasing to the eye and it's the only thing that
makes me reluctant to use this feature. I'm just weird like that. When
I close this "lookup tab", everything is back to normal (other tabs
move up, to their correct positions).