Have you ever done any arc welding ? Arcing creates very high temperatures and can melt anything close to it like a plastic fuse box. You took a very good photo...looks to me like you need to take all of your fuses out and clean them and the contact points in the fuse box. I see green corrosion on the ends. Use fine sandpaper or something...but don't short anything out. Disconnect battery first for safety. After you clean that mess up
...put some oxide inhibiting compound on them and that should stop the arcing.
This has burnt down a lot of houses also.