In Firefox, how to make middle click be exactly identical to Ctrl+Click?

Phoshi's answer using an AutoHotkey script is a nice workaround. I recommend replacing his AutoHotkey script with this modification:

SetTitleMatchMode, 2
#IfWinActive Mozilla Firefox
MButton::
Send ^{LButton}
#IfWinActive

What improvement does this make? Phoshi's script (i.e., using MButton::IfWinActive, Mozilla Firefox and so forth) will make it so the middle mouse button (outside of Firefox and other windows with "Mozilla Firefox" in the title) can only send a middle mouse click—middle-drag or middle-button-hold gestures will be impossible while the script is active. (Also, all middle-mouse-button gestures are disabled in "Mozilla Firefox"-titled windows.)

Also, SetTitleMatchMode, 2 allows the script to match Firefox even if "Mozilla Firefox" is later in the window title (otherwise it will only match if "Mozilla Firefox" is at the beginning of the window title).

MButton::
IfWinActive, Mozilla Firefox
send ^{LButton}
else
send {MButton}
return

in autohotkey would do it. Note that this turns middle click into control and left -everywhere- in firefox, not just for links.

edit: take out the additional linebreaks, I can't get them one line apart for some reason :\

If you use TabMixPlus, you can configure all mouse actions, including middle click. This works on every platform.