The screen is not updated correctly when uploading multiple files using Firefox/Linux

While uploading files, thumbnails are supposed to fade in as the image is being upladed. However, many thumbnails stays faded out after the file has been uploaded. Some doesn't appear at all.
However, at the end, it will display that all files are uploaded:

21/21  bugzilla-3.2.3-11.tar.gz

All files received.

Steps to reproduce

Try to upload let's say 21 files with size about 2.5Mb using Firefox/Linux