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From this thread, I think (having not yet tried them) that Xubuntu LTS and LXLE should meet these criteria. Almost certainly the "right" version of Debian will too, although I don't yet know which the "right" version is.
If you are dealing with users that just want it to work, I would suggest that stable is the right version of debian.
Perhaps configured to provide the latest versions of Thunderbird and Firefox (IceDove, IceWeasel).
Ben