When changing the Product Value on an existing bug the Version and Component selects are not repopulated with values appropriate for that Product. Specifically this affects #145253, which I would like to change to Fedora Extras/4/yum-utils.
Bugzilla doesnt assume that the components and versions match from one product to the next and forces the user to select the new component and version. This is by design in line with the upstream version. Please file this bug at http://bugzilla.mozilla.org against their Bugzilla product as this may end up being a good feature to have.