We've experienced similar problems - and it almost always occurs on our intel-based macs running Office 2004. I've had to download the files and open them through Parallels. What I discovered was there is an auto-fix file that runs in the PC version of Excel that doesn't exist in the Mac version. For some reason the files were corrupted upon download from Moodle and the Mac Excel version didn't have that auto-fix program installed natively in it, but again, the Windows version did. Opening in Windows Excel corrected the problem - it still left a few fields scattered (namely name and email that we just had to realign), but it was better than nothing. Oh - and we had this problem using Firefox
2.0+ (latest version).