java.net.SocketTimeoutException: Read timed out
There was one report of the same issue when using Safari (533.21.1 on OSX 10.6.8), but another user using the same OS and Safari was able to upload without problems. Users of other browsers/versions also do not have any problems.
Please verify if there is an issue with Firefox 5 on both Windows and OSX machines...and hopefully any possible solutions...
UPDATE 2011-08-23: our users that have updated to Firefox 6.0 have not run into this issue!

Thanks!
Cid