Directory upload is not implemented in Firefox, that's why you got this error message from OneDrive.

So, what are you going to do about it? I can Drag and Drop folders to OneDrive with Google Chrome! Does this mean that this is the last final reason to ditch Firefox and move over the Chrome? Or, are you no longer intending to compete?

There's the bug https://bugzilla.mozilla.org/show_bug.cgi?id=1293949 about a similar issue, which is fixed in FF50+. Could you download Beta and confirm it's fixed or not? https://www.mozilla.org/en-US/firefox/beta/all/