Any update here? Still having this issue.

This is probably something MS needs to be done on their side.

• 1763027 - outlook.office.com - drag and drop attachments not working

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

A possible workaround for now is to attach the files instead of dragging.

Same problem here. It's not a MS problem, since I can drag & drop files into Outlook when using Safari. My Firefox is fully up to date.