Firefox has been working on a number of improvements to the Firefox update experience. These improvements use new approaches to update that allow Firefox to update silently without interruption to your regular use of the browser. The update is applied on start-up as the updater needs to modify Firefox files, which may be locked while Firefox is running. A new update installation method allows Firefox to install an update while the browser is running. This means that the update can be installed immediately after it is downloaded. The new version of Firefox will launch with no delay as the update procedure has already taken place.”]
Source: https://blog.mozilla.org/futurereleases/2012/02/03/improving-the-firefox-update-experience/