The U.S. Computer Emergency Readiness Team warned on Monday that earlier versions of Firefox, and other Mozilla software based on Firefox code, contain a clutch of vulnerabilities that expose users to attack.
The Mozilla Foundation released alast week, version 126.96.36.199, which it said contained fixes for several security flaws.
According to security firm Secunia, there are a total of 21 flaws in the older versions of Firefox, such as Firefox 1.5, some of which it described as "highly critical."
US-CERT advises people who use Mozilla's e-mail software, Thunderbird, and the Internet application suite Seamonkey to also upgrade to the latest versions (Thunderbird 1.5 and Seamonkey 1.0.1). US-CERT warned that any other products based on older Mozilla components, particularly the Gecko rendering engine, may also be affected.
Firefox has traditionally been seen as being more secure than other Web browsers such as Microsoft's Internet Explorer. This is thought to be the first time that multiple vulnerabilities have been reported in Firefox and the Mozilla suite.
Secunia warned that hackers could exploit the security holes to gain control of computer systems, conduct phishing attacks and bypass security restrictions.
The vulnerabilities were discovered by Mozilla researchers, including Bernd Mielke, Alden D'Souza and Martijn Wargers, as well as by 3Com researchers working on the TippingPoint Zero Day Initiative.
This initiative encourages "responsible disclosure of vulnerabilities" to vendors, to give them time to put out patches before holes are disclosed to the public. TippingPoint started to disclose the holes to Mozilla from December last year.
Tom Espiner of ZDNet UK reported from London.