Go to input.mozilla.com, then open the findbar after the page loads. Watch the graph redraw from scratch. Close the findbar, watch it do so again.
What's "the findbar"?
That being said, some context here, the graph is redrawn on the "resize" JS event. So it redraws because what you are doing is firing such an event, and if the page is indeed resizing then the graph must be redrawn to fit its new constraints. Then this would be expected and not a bug. If your action is not in fact resizing the window, then Firefox should not fire a resize event, in which case this is a client bug.
The findbar is the toolbar that pops up giving you the ability to search the page. Ctr+F? I guess I am surprised that bringing up a chrome toolbar would have a possibility of resizing a page, since it's just an overlay.
Ah, thanks for clarifying this! This is currently expected, but your bug made me think, and it turns out we don't need to redraw on vertical size changes, only horizontal ones. So I filed bug 631102 over on Input. Since I do believe the resize JS event is fired correctly here (ergo, not a client bug), let me dupe this over to the Input enhancement I filed. It'll automatically CC you on that bug. Thanks again!