According to secunia's security advisory, several vulnerabilities were found in Firefox. Fortunately, Mozilla released Firefox 1.5.0.1 to fix them.

See the release notes and the list of security fixes.

If you still use FireFox 1.0.7, please note it is vulnerable to some of the problems as well. I'm expecting a 1.0.8 to be released but since it's taking it's time it might be easier to take the step to the 1.5 series.
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Swa Frantzen