This week, Mozilla announced and released a
series of patches for several of their products, addressing a few “critical” security flaws. Affecting Firefox, Thunderbird and SeaMonkey, the flaws could result in RSA signature forging, a system crash and memory corruptions. If you are using the older 1.5.x branch of Firefox, you'll be automatically updated to the newest version. The 2.0 series of Firefox is not affected by any of these flaws, and doesn't require patching. There aren't reports of these flaws being exploited, but keeping up to date is always a good idea.
