It's not just for browsing (FF let you close tabs and open new ones too when you middle click on a link). For instance it is instrumental if you want unix-like copy and paste, to which I am addicted by now.
Don't you just have your other hand on the keyboard all the time anyway? Set FF to open everything in tabs, so that's solved, and then use cmd+: w to close windows, q to quit, c to copy, v to paste, etc etc... they're all right there under your non-trackpad fingers.
I didn't even use middle-click back before wheel mice (when they literally had 3 buttons).
