The problem is that the malware is going to find a way through anyway. Even if it has to replace firefox.exe with a custom compiled binary that removes the sign

The problem is that the malware is going to find a way through anyway. Even if it has to replace firefox.exe with a custom compiled binary that removes the signature checking function; or simply patches out the check in the binary on your system like a warez crack. This is going to impede regular users a lot more than the advertisers.