The Mac OS X version of Mozilla shouldn't have borders around the tooltips. This will help to make Mozilla's tooltips similar to the system help tags (they'd look pretty darn close, minus the anti-aliasing of text). Also, the drop shadows remove the need for borders.
--> themes. the toolkit isn't adding the borders, it's the theme.
The Modern theme is XP and has no OS-specific styles. The Mac classic theme COULD do this, but then, it's supposed to look like Mac Classic. If we ever get an Aqua theme that would be the place to do it.
[RFE] is deprecated in favor of severity: enhancement. They have the same meaning.