Yes, using the m button is optional. The settings come from accel.cfg file in root/browser/settings folder.
# ----- Link Handling
# Middle-click links to new windows/tabs:
%ifplugin tabs
MButton = openLinkInNewTab
%else
MButton = openLinkInNewWindow
%endif
# Middle-click links to the background (instead of the above):
#%ifplugin tabs
#MButton = openLinkInBackgroundTab
#%else
#MButton = openLinkInBackground
#%endif
In your profile there is also a accel.cfg file which already had the code you want. The code is commented out. You would want to un-comment the code but not the description and save the file.
# Middle-click links to the background:
%ifplugin tabs
MButton = ID_OPEN_LINK_IN_BACKGROUNDTAB
%else
MButton = ID_OPEN_LINK_IN_BACKGROUND
%endif