It may well be off an aircraft just not a 777 by the looks of it.
Another possibility is that it was part of a rocket stage that reentered the atmosphere or never fully left. You see a lot of pressure vessels make their way back down. I suppose their shape helps them to avoid being burnt to a cinder. I would say it looks a bit too clean for that though.
Looking at the images of the washed up object and the schematic my guess would be yes it's a fire bottle but no it isn't from a 777