Command + Click not opening browser links in new tabs

Ordinarily, Command + Click will open a browser link in a new tab. This works in Chrome, Firefox, and Safari; it works on my home Mac, and my coworker's Mac. But for some reason, this hotkey is not working on my work Mac. When I try use this hotkey, links and bookmarks open in the current tab, as if I hadn't pressed the Command key at all.

My Command keys are functioning normally in other hotkeys, such as Command + O to open files.

I am using the latest, official, stable versions of Apple Mac OS X Mountain Lion (10.8.4), Google Chrome, Mozilla Firefox, and Apple Safari, on a 2012 MacBook Pro 17".

I tried disabling browser extensions, same problem.

I tried disabling hotkey apps such as ShiftIt and Quicksilver, same problem.

I haven't customized my Keyboard settings in System Preferences.

I'm using my MBP's builtin keyboard and trackpad.

Anyone else experience this?