When playing with the example page for React Drag & Drop (react-dnd), the examples at http://gaearon.github.io/react-dnd/#/dustbin-simple appear to be broken in the latest Nightly build. Firefox Nightly @ 39.0a1 (2015-03-23): Broken for me Firefox Dev Edition @ 38.0a2 (2015-03-23): Works for me Therefore I think this is a regression. Exact steps to reproduce: 1. Open http://gaearon.github.io/react-dnd/#/dustbin-simple in Firefox 2. Move the mouse over the "Glass" box 3. Keep the mouse pressed and move it towards the black/gray box with the title "Drag item here" Expected result: 1) A new hovering box with the title "Glass" should appear and stick to the mouse for the duration of the drag (that is, for the time the mouse is kept pressed during the step 3 from above) 2) The black box titled "Drag item here" should change the color to green when the hovering "Glass" box is dragged above it 3) When the hovered "Glass" box is dropped on the "Drag item here" box, a window.alert prompt should show up Actual result: - none of 1) - 3) from above.

Turns out this is only related to e10s. As e10s is disabled on Firefox Dev Edition I wasn't seeing the bug there. Removing the regression and regression-wanted keyword as this might just be a bug in e10s and set tracking for e10s.