Unless the nightlies have a different behavior than the releases, the patch level is not reported anymore. The changes went into 16.0.2 which was released on 10/26/2012. initial report on b.m.o[1] reads as follows:

  Steps to reproduce:

  1) Load http://www.delorie.com:81/some/url.txt

  Actual results:

  The User-Agent header exposes the security patch level as either a minor version
  number or as an alpha/beta/pre indicator. This data is exposed twice: in the 
  Gecko version and in the application version.

  While it is of value to expose this data to e.g. AMO, exposing it to all sites
  makes the browser more fingerprintable (see https://panopticlick.eff.org/ ) and
  doesn't serve a purpose more important than user privacy. Point releases don't
  change functionality beyond security and stability fixes, so sites shouldn't be
  sniffing the patch level anyway.

  Making trunk, alpha and beta builds look like release builds for sniffing
  purposes reduces sniffing-related failures that waste time when treated as
  functionality-related regressions by mistake.

  Expected results:
  Expected the version numbers to show the major version of the most recent 
  Firefox beta that Mozilla has shipped and not to show the security patch level 
  or an alpha/beta/pre indicator.

  Additional information:
  Internet Explorer doesn't expose the security patch level in its UA string."