When Nightly/Firefox starts, it first checks to see if an update is available then restarts if it is. This is both slow and annoying.
A better solution would be to have a shortcut/symbolic link to Nightly/Firefox, that is automatically modified to point to the new version. That way, Nightly/Firefox always starts immediately. While it's running and in use, it can check for updates and download them as a background task, storing them as a new file.
When the download is complete, the shortcut/symbolic link is modified to point to the new version so that when next started, Nightly automatically runs the new version. After starting from the new version it then deletes the old version if one exists (as part of the update process in the background task).
If updates are just "patches" rather than complete modules, then the current running version can be copied to disk, and the patches applied to the copy before changing the shortcut/symbolic link.
This approach means that no updates are actually applied at start up time, but rather done as a separate low priority background task during the current session.
The shortcut/symbolic link should be changed during the shutdown process of the current running session, so as to avoid mixing old and new versions in memory if the OS decides that it needs to reload code from disk due a to a page fault.