Scroll bar vibration onContextKeyPress in FF

I pressed the context menu button in Firefox with the cursor over a misspelled word, and the scroll bar (and browser page) went up and down over and over, about 20 or 30 times, before settling down again. It didn't jam, I tried the Start Menu and Firefox was still vibrating in the background. I had run virus scanners on it, but maybe I should update and try again? Or is this a FF bug? I tried it in explorer windows and it works as usual. I've used this button this way in FF before on this computer with no problem like this.