Drag and drop NOT working with Gmail in Firefox, on Ubuntu 18.04

Dragging and dropping files in Gmail is not working in Ubuntu 18.04 and Mozilla Firefox.

I trap the file, drag it to the mail box (drop your file here) and nothing happens (The image of the file returns to its original place. Sometimes the "drop your file here" Gmail box remains frozen) The file never is attached.

It happens to me too, only in Gmail, weird that it doesn't happen in Google Drive.

A workaround I just found is to make a fast return to the window from I'm getting the file while dragging to a Gmail's message, then drop the file in the Gmail's message area and it will be attached as expected. Look at this video.

Here is what works for me: while holding the mouse button down, drag the cursor into the compose pane, then outside the Firefox window, and then reenter slowly from the bottom part of the compose pane. Once the cursor turns into a hand with a "+" sign, you can drop the file(s). This way it works 99% of the time.

It usually doesn't work if you reenter from the sides, or from above, or from the bottom part of the window but outside the compose pane.

You can also try directly entering the Firefox window from the bottom of the compose pane without doing the exit/enter voodoo mentioned above, but it works maybe 50% of the time.

Super weird, I know...