Browser Diligency
A recent study has shown that 83.3% of firefox users are running the latest version, whilst only 47.6% of IE users are running the latest patched version. The report suggested that legacy entrprise applications maybe to blame. Some legacy software don’t allow software updates. But I’d throw in that firefox can download and install the latest version for you, which doesn’t appear to be the case for IE.
But look here Microsoft claim IE8 is more trustworthy now! after they have just fixed a code injection problem with vista.