The update, which started being sent out at the weekend, fixes flaws that security firm Secunia described as 'highly critical'.
The flaws affect Firefox 1.5 and 2.0 for Windows, Mac and Linux systems.
"Due to the security fixes, we strongly recommend that all Firefox users upgrade to these latest releases," said Mozilla in a statement.
The fixes include a location.hostname vulnerability that independent Polish coder Michal Zalewski found a few weeks ago, and a critical memory corruption bug that can cause the browser to crash.
This could be the last security fix for Firefox 1.5, since the organisation will stop supporting the old browser on 24 April.
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