Both Firefox and Chrome operate on a six week release cycle on average.  Additionally they only tend to do mid release deployments when it's a major issue.  That said, Google only supports the most current release of Chrome (which is reasonable since it auto-updates).  Mozilla on the other hand supports the most current release of Firefox and the most current Long term release.

Microsoft will not support Windows XP past 2014.  It's in extended release support as is.  Once support stops, given MS's track record it will begin to suffer security issues.  This is why they are urging people to upgrade to Windows 7 / 8.