The only patch that landed is bug 707654 between the 2012-01-22 and 2012-01-23 builds.
Whenever focus lands on a focusable item within an iframe, NVDA's virtual buffer and a lot of other IAccessible2 related functions like getting the ancestry chain of an accessible, break. For example when pressing CTRL+L, NVDA no longer says "Navigation toolbar, go to a web site edit combo". It will only say the last part about the "go to a website".
Sometimes, going toa web site without iframes recovers things, but most often Nightly has to be restarted in order for things to start working again.
Each page that embeds the "Tweet" or "Like this on Facebook" iframes breaks as soon as one navigates into it with NVDA's virtual buffer. GMail is also broken.
(For the benefit of anyone else experiencing this, who might not have seen the IRC chat).
Bug 707654 has been backed out of m-c (in https://hg.mozilla.org/mozilla-central/rev/9b069a37f58f) and new nightlies triggered, so hopefully this should be fixed once they are complete.
Verified fixed in second nightly build from January 23: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0a1) Gecko/20120123 Firefox/12.0a1