[MajesticHobo]

Okay. So it's a protection against browser exploits, not overreaching web APIs.

[koolba]

Good security has layers. That way if one falls through, hopefully the next layer will catch it.

[cm3]

It's more a boundary for the browser not to pass because it has no business. I've seen an alternative approach which used a separate user account for Firefox and then SSH forwarding of X.