[BitBucket user: Sébastien Jodogne]
[BitBucket date: 2018-12-28.08:03:15]

Maybe linked to this issue? https://askubuntu.com/a/981426

On Ubuntu 14.04 with Firefox 60.0.1, using your new "upload.html" page, uploading 1000 DICOM files with the "Browse..." button is fast and perfectly usable.

In either case, I am not in favor of blocking drag-and-drop depending on the browser, as this issue is visibly independent of Orthanc, and could be fixed in future releases of Ubuntu and Firefox. Furthermore, maybe this issue is very specific to Ubuntu/Nautilus: KDE-based distributions might be unaffected. Finally, Firefox on Windows is apparently not affected, so disabling drag-and-drop on any Firefox is definitely not desirable.

If this popup warning is too intrusive, I would simply recommend to add a textual warning at the top of the upload form, possibly only displayed if the browser is Firefox.