Drag'n'drop bug in Firefox 49+ on Linux

Platform

OS

Linux (Kubuntu, Ubuntu, Fedora)

Firefox version

Atleast 49.x and 50.x

Extensions

Atleast one extension installed. Extension must be the kind that requires Firefox to be restarted when enabled/disabled.

How to reproduce

1. Go to http://petrit.net/dragndrop.html
2. Select some text on some other application than Firefox. For example a texteditor.
3. Drag the text and drop it on the yellow area on the web page.
4. The page shows the content of event.dataTransfer.types in textarea. And the values of event.dataTransfer.getData(type) for types text/plain, text/html and text on other textareas.

What should happen

Types should include text/plain (and possibly text/html depending on where the text is dragged from), and the dragged text should be shown in the textarea for text/plain.

What happens

The "types"-textarea shows only the types application/x-moz-file and Files as if some files were dragged. Other textareas are empty.

Other

Restarting Firefox in safe-mode, i.e., all extensions disabled, does not help, if those are normally enabled. Only really disabling them helps.

Dragging text from inside the Firefox works always as it should.