I was lamenting the fact that I missed being able to middle-click a link in Firefox on OS X and have the link open in a new background tab. You know, like it did on my Windows box. Dan overheard this and told me about customizing the button behaviour using the Microsoft (yes, Microsoft) mouse control panel. Sure enough, I added the Firefox.app and assigned the middle button to Cmd-Option-Click. All better now, thank you.