Are you blocking scripts?
I went to http://oldversion.com.
I scrolled down to "Internet" and clicked on "Mozilla Firefox"
On the next page, I selected a version and clicked on it
On the next page, I clicked on the green "Download Now" button, and followed the prompts...
Since I don't need another copy of Firefox, I aborted the download.
If you are running the "noscript" plug-in, or are otherwise blocking scripts, the "Download Now" button won't work, unless you give oldversion.com permission to run scripts.