I've just changed corruptedContentError.longDesc in
mail/locales/en-US/chrome/overrides/netError.dtd on comm-central trunk.
It wasn't possible to change the entity because that's shared across the
Mozilla codebase.
As far as I can tell, there are 9 locales affected: ru, de, pl, zh-TW,
cs, lt, fr, es-ES, es-AR. Sorry for wasting your time.
Steffen
https://bugzilla.mozilla.org/show_bug.cgi?id=663608
http://hg.mozilla.org/comm-central/rev/6d46dca5b7cf