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.
Time flies like an arrow
Fruit flies like a banana