Though this feature may have existed before, as of Build 2002012304, clicking a link with the mouse button in Windows opens the link in a new tab. Or the other handy trick is to "ctrl-click" on a link.
Better yet, this feature by default will open a link in a new window instead of using those annoying tabs!
This doesn't work in Moz 1.4 and 1.5 (maybe even earlier) on Mac OS X. i have to get USB Overdrive and map middle click to CMD+click. Carp!