User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.143 Safari/537.36 Steps to reproduce: With the following minimal reproducible example: https://output.jsbin.com/dicote 1. Load the previous URL in Firefox 32bit application 2. Drag & drop a folder from the desktop to the browser window Actual results: The dropped folder was read erroneously by *FileReader.readAsDataURL*. The following log was displayed: Dropped item: <FOLDER_NAME> (no type available) Event log: + load ... ⚠️ Item content (50 first characters): data:application/octet-stream;base64, Expected results: *FileReader.readAsDataURL* should trigger an "error" event with the dropped folder. The following log should be displayed instead: Dropped item: <FOLDER_NAME> (no type available) Event log: + error ... ✅

The bug is also present in Firefox Beta (50.0b) and Nightly (52.0a1).

Did it use to work in previous versions of Firefox?

Fix range: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=b2409b36cd07bba045bf38dd131166c36ea3da4a&tochange=3d0aef4586bdbd81591811ec627c85161fc11065 It works in FF50+ with e10s disabled after bug 906420 which implemented DataTransferItem. The only issue remaining is with e10s enabled but there are some known issues with drag&drop in e10s mode (like bug 1311610). Michael, do know if there is a bug to track this issue with e10s enabled?

I created bug 1312120 as a metabug.