As it is right now: many dictionaries will break once people are told to upgrade to Thunderbird 60.
This will make many people unhappy as having working dictionaries is a central part to writing emails :-D.
Since there is no warranty dictionary maintainers will update in time and dictionaries are… Just dictionaries: would it be possible to drop the “compatible with 60” check only from dictionaries? Or is there really no distinction whatsoever between dictionaries and add-ons?
The way I see it: I could expect an add-on to break, but I wouldn’t expect dictionaries to do so (was certainly surprised now that I wanted to switch to another dictionary and it wasn’t available).
I am temporarily disabling the extensions.strictCompatibility setting to continue writing emails that don’t make other people’s eyes hurt :).