profiles: firefox-common: add dbus comment for drag and drop

This entry enables drag and drop from Dolphin.

Fixes netblue30#6444.

Reported-by: @Utini2000
Suggested-by: @rusty-snake

Author: kmk3

etc/profile-a-l/firefox-common.profile

@@ -100,6 +100,8 @@ dbus-system none
 #dbus-user.talk org.freedesktop.Notifications
 # Add the next line to firefox-common.local to allow inhibiting screensavers.
 #dbus-user.talk org.freedesktop.ScreenSaver
+# Add the next line to firefox-common.local to enable drag and drop in Dolphin.
+#dbus-user.talk org.freedesktop.portal.Documents
 # Add the next lines to firefox-common.local for plasma browser integration.
 #dbus-user.own org.mpris.MediaPlayer2.plasma-browser-integration
 #dbus-user.talk org.kde.JobViewServer