IIRC, in the case of Firefox the problem was not porting Firefox itself to 64-bit. That was done a long time ago. The problem was all of the 32-bit dependencies

IIRC, in the case of Firefox the problem was not porting Firefox itself to 64-bit. That was done a long time ago. The problem was all of the 32-bit dependencies people had installed. For example - if a user only had 32-bit Flash installed and they updated or installed 64-bit Firefox, they'd be annoyed that Flash "broke". That might not be the best or most current example, but it's that kind of thing that held up 64-bit default on Windows.