In this update, Mozilla claims to have fixed several security issues and fixed several stability issues, somehow it seems to be the very familiar wording.
In detail, it addressed following:
MFSA 2011-49 Memory corruption while profiling using Firebug
MFSA 2011-47 Potential XSS against sites using Shift-JIS
MFSA 2011-46 loadSubScript unwraps XPCNativeWrapper scope parameter (1.9.2 branch)
UserAgent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.23) Gecko/20110920 AlexaToolbar/alxf-2.14 Firefox/3.6.23
