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Clarified legacy add-on support period
Mozilla core have disabled "legacy" add-ons (not based purely on WebExtensions) in Firefox 57. Because this type of add-on is not supported from mozilla57 any more, many interfaces have been removed from Mozilla core.
However, Thunderbird 57-59 60 and beyond and Seamonkey '''still support''' these add-ons if the appropriate modifications are done to adapt to the changed interfaces. In general, Thunderbird is adding WebExtension support ({{bug|1396172}}) while maintaining "legacy" add-ons and hybrid add-ons.
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