Jason, I did a complete body off rebuild of my car so the engine, as well as everything else, was out. I wouldn't even consider doing that with the engine in. As it was, it was pretty messy because of the slope of the firewall. Replacing the firewall from the inside with the engine in seems doable though.
Alex (4129R), I think what I was thinking is that I wanted to get rid of the horsehair insulation and I needed something solid to mount the new insulation on. That and replacing the entire firewall seemed like overkill - it's just as likely that I was intimidated by the prospect. I don't think it was till later that I read about people using marine plywood. I think that is a much better idea than what I did. Luckily, my fiberboard was not in such rough shape that I absolutely had to replace it.