Please add the middle mouse button prefs (created in bug 24571) to the prefs window. Some users who use both Windows and Linux will be frustrated when they can't figure out how to make Mozilla do neat things with the middle mouse button, and conversely, many of them would be very happy to find the prefs.
cc blake in case he's interested in doing this.
See also bug 55705, [rfe] enable middlemouse.openNewWindow by default on Windows
taking this if Matt doesn't mind.
*** Bug 60596 has been marked as a duplicate of this bug. ***
I think this is no longer necessary now that bug 55705 is fixed. (Middle click now opens a link in a new window by default on Windows.)