pages not loading and attempting to download files

Whenever I try loading my site https://www.dukapress.org in FireFox 3.6 and Safari 5 on Mac, more than half the time, pages download as a pagename.gz file, rather than displaying in the browser.

I tried to see if I could find the error in the page headers that was causing the issue, since both of these browsers support gzip compression, but am still unsure.

What could be wrong?