To reproduce (Nightly 43.0a1 x64 2015-08-31, in win10 x64) - open an e10s window, and a new tab (about:newtab) - open Network Monitor (via menu button or via Ctrl+Shift+Q) - open any web page, eg. https://www.mozilla.org/en-US/ You'll see that the Network Monitor immediately closes. This only happens in a e10s window. Also, there isn't any relevant entry in Browser Toolkit|Console when this occurs.

I am pretty sure this is the same as bug 1068400, so let's depend on it for now. The issue is that while you may be using an e10s window, about:newtab forces itself to be loaded in main process for the moment, while navigating to random websites does load in the content process. The toolbox does not handle this process transition. It will effectively be "fixed" soon, as about:newtab will soon load in the child process as well.

Correct. And this is the case for any tool, not just the network monitor.