sorry big, i was just joshing..
just like james said, you can open the default accel.cfg and hashout what you don't want and unhash what you want
you can also go to edit>configuration>accelerators
the accel.cfg file should open in your notepad, just add the following script at the end of the file:
# Middle-click links to the background:
%ifplugin tabs
MButton = ID_OPEN_LINK_IN_BACKGROUNDTAB
%else
MButton = ID_OPEN_LINK_IN_BACKGROUND
%endif
save the file.
this should trigger background tabs from middle click.
another and easier way to do that is just go to tools>mouse accelerators> and select sea monkey
also check this thread:
http://kmeleonbrowser.org/forum/read.php?1,81813