The Mozilla foundation has released 1.0.5.7 to patch security issues. Autoupdate should install this for most users automatically.
Mozilla Firefox 1.5.0.7 -- Release Notes
http://www.mozilla.com/firefox/releases/1.5.0.7.html
Mozilla Firefox 1.5.0.7 -- Security Changes
http://www.mozilla.org/projects/security/known-vulnerabilities.html#firefox1.5.0.7
Mozilla Firefox 1.5.0.7 -- Download Site
(if autoupdate is not enabled)
http://www.mozilla.com/firefox/
Mozilla Firefox 1.5.0.7 -- Secunia Information
http://secunia.com/advisories/21906/
QUOTE: Some vulnerabilities have been reported in Mozilla Firefox, which can be exploited by malicious people to conduct man-in-the-middle, spoofing, and cross-site scripting attacks, and potentially compromise a user's system.
1) An error in the handling of JavaScript regular expressions containing a minimal quantifier can be exploited to cause a heap-based buffer overflow. Successful exploitation may allow execution of arbitrary code.
2) The auto-update mechanism uses SSL to communicate securely. The problem is that users may have accepted an unverifiable self-signed certificate when visiting a web site, which will allow an attacker to redirect the update check to a malicious web site in a man-in-the-middle attack.
3) Some time-dependent errors during text display can be exploited to corrupt memory. Successful exploitation may allow execution of arbitrary code.
4) An error exists within the verification of certain signatures in the bundled Network Security Services (NSS) library.
5) An error in the cross-domain handling can be exploited to inject arbitrary HTML and script code in a sub-frame of another web site via a "[window].frames[index].document.open()" call.
6) An error exists due to blocked popups opened from the status bar via the "blocked popups" functionality being opened in an incorrect context in certain situations. This may be exploited to execute arbitrary HTML and script code in a user's browser session in context of an arbitrary web site.
7) Some unspecified memory corruption errors may be exploited to execute arbitrary code