Yesterday, Richard Gemmell left the following comment on my blog (I've trimmed to the critical part):
I was referring to the way that IE can be tricked into calling the Firefox command line with multiple parameters instead of the single parameter registered with the URL handler.
I saw this comment and was really confused for a second, until I realized the disconnect. The problem is that *nix and Windows handle command line arguments totally differently. On *nix, you launch a program using the execve API (or it's cousins execvp, execl, execlp, execle, and execvp). The interesting thing about these APIs is that they allow the caller to specify each of the command line arguments - the signature for execve is:
In *nix, the shell is responsible for turning the string provided by the user into the argv parameter to the program.
On Windows, the command line doesn't work that way. Instead, you launch a new program using the CreateProcess API, which takes the command line as a string (the lpComandLine parameter to CreateProcess). It's considered the responsibility of the newly started application to call the GetCommandLine API to retrieve that command line and parse it (possibly using the CommandLineToArgvW helper function).
So when Richard talked about IE "tricking" Firefox by calling it with multiple parameters, he was apparently thinking about the *nix model where an application launches a new application with multiple command line arguments. But that model isn't the Windows model - instead, in the Windows model, the application is responsible for parsing it's own command line arguments, and thus IE can't "trick" anything - it's just asking the shell to pass a string to the application, and it's the application's job to figure out how handle that string.
We can discuss the relative merits of that decision, but it was a decision made over 25 years ago (in MS-DOS 2.0).
 Yes, I know that the execl() API allows you to specify a command line string, but the execl() API parses that command line string into argv and argc before calling execve.