In Seamonkey, type about:config
in the address bar. Type middle
in the filter bar to narrow down the selection, then change middlemouse.contentLoadURL
to false. That allows middle click (mouse wheel click) to close tabs.
For the close buttons on each tab http://tom-cat.com/mozilla/extensions.html#seatabx
This addon may or may not work with your version of Seamonkey though.
There is an about
:config option (browser.tabs.closeButtons
) but this has no effect in Seamonkey, only Firefox.