I have recently reported a bug on Microsoft Connect about the problem that pressing F1 in Visual Studio 2013 to open the MSDN help page always opens a new Firefox window instead of a new tab (see here for the Connect entry).
Microsoft replied that this is a bug in Firefox, but I can't reproduce it with any other application. I've created a small C# program that starts Firefox with Process.Start() using the exact same command line arguments that are shown when Visual Studio 2013 starts Firefox (I've got them with the Process Explorer tool), but that also doesn't reproduce the issue. I can see that Process.Start() uses two different Windows APIs based on the UseShellExecute property, but both ways don't reproduce the issue.
So my question is: How exactly does Visual Studio 2013 internally start Firefox? I know that it's not simply invoking the HTTP protocol handler since that uses different command line arguments for Firefox, so it must be something internal. If I can reproduce this with a small sample, hopefully the bug can be fixed within Firefox.