As the Firefox installer now nicely upgrades and doesn't leave the old one in the registry, there is no need to differentiate versions in "package id".
When a new version comes out, replace every occurrence of the version number in the installer file from, e.g. 1.5.0.4, with, e.g. 1.5.0.5 and then increment the package revision number.
Note that the Add/Remove Programs string of Firefox 1.5 is used for all subversions (1.5.0.4, 1.5.0.5 etc). We don't bump the package version number assuming that minor version upgrade works from Firefox itself. If we did, we would force reinstall on the people who already have Firefox automatically updated.
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