There is currently an issue with the downloads working on Safari. The web team is aware of it and working on fixing the issue. Please try in another browser and report if that does not work. I will update this posting when the Safari download issues are fixed.
I just tried it, no issues downloading at the moment.
Both the dmg and tarball types of 6.3.3 UF downloaded without issue.
Where are you originating from? When was there an issue downloading it? Might be useful information to help our IT team determine if there was an issue with the site recently and/or an issue regionally.