Just like Internet Explorer, Firefox has security holes. Unlike Internet Explorer, any holes which are discovered tend to be patched up flippin' fast. Well, I say patched, but actually it's another download and reinstall required, and at the time of writing, Firefox 1.0.4 isn't localised to en-GB yet, but if you want to play on the safe side, you can grab the updated version now.
[Mozilla Firefox Website]
