Maybe this is just how browser works (Firefox), but every time i cancel a download in FDM (after it is redirected to FDM with FDM add-on in Firefox) it closes FDM dialog, but then browser opens a Save as dialog to save a file. It is a bit annoying having to close that window every time. Maybe FDM can suppress that.

fdm>setting>untick "use browser of u cancel download via fdm".

if already untick, then its browser own problem...go report to mozilla instead.