This page exposed a Canvas-related bug in FireFox and in IE9. Initially there is a gray SVG document with a red circle. After clicking the Create Canvas button a Canvas object is created with a red border and a translucent green background. This Canvas object is layered above the SVG object and inset by 10px. When you then click the Braw Into Canvas button a blue line is drawn into the Canvas.
On FireFox 7.0.1 and FireFox Nightly and IE9 when the ctx.stroke() function is called to complete the line the background of the canvas is erased and becomes transparent.
This only replicates part of the problem described here: Canvas-related bug in FireFox. When starting the model in this page Avalanche-JS Demo no lines are drawn in the graph on the right while the model is running. This is the time that the Canvas object is being draw into.