Automatic updates for Firefox 10 Nightly Windows (x32 and x64) builds had been temporally disabled earlier this week due to start up crashes with ‘Windows 8’ [Bug 695161]. It appears this issue has been corrected on 10/20/11 and quite possibly by the time this article is published, users will be able to update via Help > About Nightly. If the automatic updates are still not functioning you can always update manually by downloading the latest trunk build here.