Try to download here: http://releases.mozilla.org/pub/mozilla.org/firefox/releases/5.0/win32/en-US/

I had this trouble too... it seems that the British version for XP (and maybe others) is corrupt. I got round it by going to

http://www.mozilla.com/en-US/firefox/all.html

and downloading the English (US) version instead.

Thank you people the links helped and it seems that the original link is fixed now.

Just had the same problem on a different PC - I tried my answer above and it didn't work... However after I did the following, it did work:

I cleared the firefox browser cache (ctrl-shift-del). I then deleted the downloaded firefox.exe from my downloads folder. I then downloaded the US version (as above). I then stopped firefox, and ran the firefox.exe file from my downloads directory.