Can't seem to properly download rom

I'm trying to download Pokemon Leafgreen. It downloads, but the only valid option I have to save it as is to open with Firefox, which doesn't work. How do I change it? Not sure if I worded that correctly, but I hope I got my message across. :)

I think you mean this: https://www.romulation.org/forum/index.php/topic,3880.0.html