You can inspect the source code via the MXR site.

• http://mxr.mozilla.org/

I'm beginning to think the build process differs for each OS, because I searched that site for browsercomps.dll but didn't find it. Instead, I found a lot of separate components. Please help me find the component that does the "private" action so I can override it.

Try to search for private-window.
You will likely want to check the mozilla-release branch.

• resource:///components/nsBrowserContentHandler.js

Interesting: I commented out the if statement that checks the command line for "private-window", and nothing changed.

C:\Program Files\Mozilla Firefox\firefox.exe -private-window

loaded with a private window anyway. I'm suspecting that the return value of the handleFlag function is a false negative, and that the program has been patched somewhere else to work around the bug, but I'm not certain. As a matter of fact, no statements that I put inside that if statement ever execute. I could probably completely remove the if statement with everything in it, and the private window would still open. This is supremely odd, well near spooky!