Hi Steve,
I'm running Windows 7 and FireFox 33.0.2, but wasn't able to recreate the problem. There are a couple of things you could try:
- The upload system uses Flash, try upgrading to the latest version of it.
- Try uploading to a none CMSB site, and see if you have the same problem.
- Try disabling any plugins you have running in Firefox one by one, and attempting an upload.
Let me know if these help identify/resolve the problem.
Greg
Greg Thomas
PHP Programmer - interactivetools.com