Firefox 56.0.1 (64-bit) in safe mode on FreeBSD-CURRENT, Dolphin, KDE Plasma 5, KWin.
Dropped: ten different files. Firefox produced ten tabs for just one of the files.
Can anyone reproduce that, or other misbehaviours, with drag and drop from (for example) Dolphin?
I wonder whether KWin is a factor …
Cross reference: