I also had this problem with Firefox 3.0 and 3.5 too. Basically, I've never been able to get Firefox to auto-update. It will download the small 2 MB or so file and fail upon restarting and attempting to install the update. It then tries to download the full 11 MB update and also fails. This has happened with every build of 3.6 including the most recent 3.6.12. I can download 3.6.13 and install manually with no issues however. I'm a UK resident and use the En-GB version of Firefox. I can't help wondering if the problem arises because Firefox is attempting to download and install the En-US version? I noticed that the full download for the En-GB version is 8 MB yet the auto-update when it attempts to install the full version is almost 11 MB! It's annoying that the browser's own updater fails to work as it should not be necessary to manually download and install updates.