To reproduce the bug:

1. Double-click on the first word (Double) of this line; do not release the mouse button.
2. Move the mouse pointer upward to the word reproduce.
3. Move the mouse pointer downward to the word Double.
4. Move the mouse pointer to the right.

Most often, the word Double is not selected. Reported as Mozilla bug 677438.

Reproducible at least from Firefox 5 to Firefox Nightly 61.0.1.