To open a link in a new tab rather than a new window,
set your target to "_new" instead of "_blank".
This works with Netscape and Firefox, but unfortunately not with IE.
[EDIT: Beware of this misleading tip:
While often confused with "_blank", "_new" is in fact not a special value; it's just a window named "_new". Therefore, other links opened with target="_new" reuse the same tab/window and will NOT open a new one. Probably not what you intended.]