If you are running mozilla firefox 1.5.X, you should receive an automatically update from mozilla browser. if not, you can go Firefox main site to download the latest firefox version.
This update is just released two days ago and firefox update purpose is resolve or fixes a security issue that was found early last week.The security issue was a denial of service vulnerability. This was another quick fix by Mozilla while Internet Explorer continues to get hammered with critical bugs.
Here is the release note information that mention about update purpose :- Mozilla firefox 18.104.22.168 release notes