When you middle click a link to open a tab, the focus should stay on the same window instead of switching to the newly opened tab.

you can open new tab without focus(in the background) by doing this:
click on tools menu> mouse accelerators> select 'seamonkey'
restart k-meleon

alternatively, you can also do this to get the same result:
click on edit menu>configuration> select accelerators

accel.cfg should open in your default handling notepad for cfg files:
add this to it at the bottom:

%ifplugin tabs
MButton = ID_OPEN_LINK_IN_BACKGROUNDTAB
%else
MButton = ID_OPEN_LINK_IN_BACKGROUND
%endif

restart kmeleon