[asutherland]

Clarification for those who parsed this (incorrectly) like I did: localStorage does work in Firefox in Private Browsing Mode[1], it's IndexedDB and the DOM Cache API that throw. (Although Firefox has private browsing support for IndexedDB in the works. That effort is tracked on https://bugzilla.mozilla.org/show_bug.cgi?id=781982.)

1: But it's exclusively memory-backed and will be forgotten somewhere between 0 and 20 seconds after the last active page for the origin is gone. It is not persistent for the lifetime of the private browsing session. However, because Firefox has a back-forward cache ("bfcache") that keeps pages around (frozen) for a while after you navigate the window to another location, the storage may be kept alive for some time even with no active windows.