I try to drag and drop a file from nautilus to firefox but can't do that.
2 Answers
Go to Preferences > Views > Experimental Is "Use the new views" checked? If so, uncheck it and you should be able to do drag-and-drop.
Note that, even with the "new views" enabled, drag-and-drop is still possible from the "list view", but not from the "icon view".
edit In recent versions of Nautilus, the 'Experimental' section in the preferences as disappered, and the "new view" seems to be the only one available now, :-/ Note however that it is only for the "icon view". As stated above, switching any Nautilus window to "list view" enables drag-n-drop to other apps. If you want to do it from the Desktop, you have to open it in a window ("Show Desktop in Files").
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In my case, the issue was having the dragged files on a network drive (smb). Copying them locally before drag helped.