I was only to get this bug to happen once. I've been doing testing in Nightly, and have hit an unknown bug where, if the Browser Toolbox is open, and I close Nightly without unpausing the toolbox debugger, Nightly hangs, and I have to use 'force quit' to close it. I went through the 'force quit' cycle many times, probably tens of times, and then I started consistently seeing an error when I'd try to open the Browser Toolbox (see attached image). I tried opening Nightly from the command line, and didn't see anything when I opened the toolbox and saw the error, but when I closed the toolbox, I saw this trace: JavaScript error: resource://gre/modules/PerformanceStats.jsm, line 208: NS_ERROR_NOT_AVAILABLE: Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIPerformanceStatsService.isMonitoringJank] JavaScript error: resource://gre/modules/PerformanceStats.jsm, line 492: Error: forget() called twice So, summarizing: STR: 1. Open nightly 2. Open the browser toolbox 3. Pause the toolbox debugger 4. Close nightly 5. Toolbox process hangs 6. Force Quit to kill the Nightly process (via right-click on Nightly icon in Mac OS X dock) 7. Repeat steps 1-6 many times 8. Open nightly via command line 9. Open browser toolbox 10. See an empty window with the contents shown in the screenshot 11. Close browser toolbox 12. See the traceback pasted above This is probably some kind of transient thing caused by failing to clean up resources over and over, but thought it couldn't hurt to file the bug anyway.

I experienced a similar issue with an open crash reporter window in the background. Dismissing it seemed to clean up what was necessary to get the browser toolbox to run again.