Drag and drop of folder using FF58 on NC13RC2 does nothing. No error, just nothing. Drag and drop of individual files, or multiple files, works as expected. Chromium works fine.
Will test on earlier versions of NC also. Certainly FF has worked in the past, pre-Quantum.
Can confirm that with FF58 and NC12 folder drag and drop works as expected.
Can you open a bug report on Github?
Developers are more likely to see your problem description there. Please make sure to use [NC13 RC3] in the title. It helps a lot.
I was hoping someone would come along and tell me I’d got some silly FF setting that was causing the issue! Bug reported https://github.com/nextcloud/server/issues/8093