Very easy to reproduce:
1 Drag any curve point in a besie curve editor
2 While still dragging press 'Esc' button.
FF Crashes with the following exception.
<class XFW::StateError> Unable to start editing session for control 00000000('<null pointer>') because editing session for control 2B36DC08('<class Controls::CurveEditorControl>@2B36DC08') is still in progress.
PS
For this bug I want to thank my friend who succeeded to place his butt right on the keyboard while I was editing a bessie curve in a very awkward spot on the flow.