In FF, the file upload dialog does not update the loading bar after a file is successfully uploaded

Reproduced in Firefox.

Steps to reproduce:
1. Run Confluence 5.8
2. From Firefox, create a new page
3. Upload an attachment to the page
4. Notice that the upload loading bar does not disappear
5. Close the dialog and reopen - the file thumbnail will appear as though the file is successfully uploaded

See attachment for what screen the file gets stuck on.