if the uninstaller detects a previous installed version of Mozilla 'somewhere', the installer should ask if it should uninstall that (old) Mozilla first. another option: the installer looks only at the directory the user wants to install to. this is very useful for the users that are following the recommended builds at www.mozillazine.org/build_comments, it reduces the number of mouse-clicks for installing a new build of mozilla significantly. personally, i didn't know that i should uninstall mozilla before installing a new version, it has always worked. in bug 107971 this was pointed out to me (not that it made any difference regarding that 'bug', however)
The installer is designed to upgrade an existing installation: WONTFIX. This works reliably in milestone builds (which get more testing and polish), but occasionally the nightlies are broken as developers make changes requiring install clean up and fail to tell the install team.