As others may have mentioned, it's because the binaries are overwritten. But Ubuntu used to ship a plugin with their builds, xul-ext-ubufox, that would detect t

As others may have mentioned, it's because the binaries are overwritten. But Ubuntu used to ship a plugin with their builds, xul-ext-ubufox, that would detect this and prompt for restart on upgrade, but for some reason it's disabled now according to the package description: This package currently ships no functionality, but may be used again in the future to display a restart notification after upgrading Firefox.