getThunderbirdVersion().major >= 91to check the version.
headerNameused for the styling header entries has been renamed to
message-header-label. The class
message-header-rowhas been added, styling the entire row.
msgHeaderView-buttonused to style toolbar buttons has been renamed to
getItem()method has been changed to return one item instead of an array.
calIOperationinterfaces are still used in various places but the general direction is to remove them eventually after 102 in favor of Promises and ReadableStreams. If you have code that uses them internally, please update where feasible.
_cachedAdoptItemCallbackproperty on your provider class. This is an unfortunate hack needed to maintain the order the "onAddItem" event is fired by
calCachedCalendarsets this property in the
doAdoptItem()method and it should be called by your provider just before returning in the
adopItem()method. An example of this can be seen here in the ICS provider.
BaseClassprovider has a default implementation however providers not extending it should provide their own implementation.