Steps to reproduce: 1. Click on this page to make sure it's focused. 2a. Hover your mouse over some word in this comment press Cmd+Ctrl+D, or 2b. Three-finger-tap a word in this comment with your touchpad. Expected results: The dictionary popup should appear and show content. Actual results: The dictionary popup appears and stays blank. I broke this in bug 1354715.
Fixed by backing out bug 1354715. Should be in tomorrow's nightly and 54b4.