I use Firefox 4.0.1 without any problem downloading aircraft. I wonder if your problem is your file expander. I am currently using 'Stuffit' ( 13.0.3) and have that set as my first choice to unzip files.
I have just looked at my version of Stuffit, and I am informed there is a newer version waiting to be downloaded (14.0.1) so I shall install that today. It may be worth a try for you too.
If that doesn't prove successful, another free for Mac app is called GUI Tar. This seems to be able to handle everything that Stuffit can't.