When new version of SeaMonkey starts to provide the mentioned addons in the "app-global" location, the copied ones of the previous version still remain in the user profile (and since it has the highest priority, still take effect).
Fortunately, each distribution addon should have the correspond extensions.installedDistroAddon. preference, which means that the addon is not installed by the user (manually), but just was copied automatically from a system location.
The patch adds a proper check for this case at the application's update time, and removes the old copies of such addons from the user profile.
Additonally, it allows user to have a more recent version anyway, as well as preserves the disabled states.
Regardless of the acceptance of the proposed change upstream, the patch will be beneficial enough for downstream (ie. Linux), since it is too easy to place addons "just in another dir" in the distro packages.