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 wind

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).