When I use middle click to open a new link, I prefer for it to open in a new background layer. The reason being I would from the orignal page open a few links each on a new layer. But if the new layer is in the foreground I would then have to go back to the original layer and open it again. I know there is an "open in background layer" in the context menu, but I would prefer to use middle mouse click for less clicking. Is there a config for that?

go to Edit > Configuration > Accelerators

and modify lines to look like this..

# Middle-click links to the background:
%ifplugin layers
MButton = layers(OpenLinkBg)
%else
MButton = ID_OPEN_LINK_IN_BACKGROUND
%endif

Maybe also have a look at Tools > MouseShortcuts > Firefox / SeaMonkey