I wrote documentation for the new support for specifying a local pathname string when instantiating a DOM File object today. Then it was pointed out that feature is on mozilla-central, not Aurora, so I’ve now created Firefox 6 for developers. So we’ve already, apparently, started documentation for Firefox 6.
That means it’s time to remind you to add links there if you document anything for Firefox 6. But, realistically, in most cases you’ll want to focus on stuff for Firefox 5 for now.