I just recently noticed that anytime Mozilla releases a new version of Firefox and the browser tries to automatically upgrade itself, it fails.
After spending a few minutes looking at this I discovered that the Kaspersky Internet Security 2009 software is somehow blocking the installation. If I disable Kaspersky and then manually kick off the upgrade from the browser toolbar, Help -> Apply Updates the update installs fine.
There's always some little bug and/or problem somewhere. It's either too much security or too little security.
Cheers!