Computer Configuration > Administrative Templates > Firefox > Content Analysis (DLP) > Interception Points > Drag And Drop
Both
HKLM\Software\Policies\Mozilla\Firefox\ContentAnalysis\InterceptionPoints\DragAndDrop
If this policy is disabled, all formats will be analyzed in what is being dropped, which some DLP agents may not expect. If this policy is enabled or not configured, only the text/plain format will be analyzed in what is being dropped.