Changes

'''Legacy add-ons''': Despite Mozilla core interface removals, Thunderbird 57-60 and beyond and Seamonkey will '''<u>still support</u> legacy add-ons <u>if</u> appropriate modifications are done to the add-on to adapt to the changed interfaces'''. Core interface changes and removals have come because Mozilla core Firefox platform have disabled "legacy" add-ons (add-ons which are not based purely on WebExtensions) in Firefox 57. Beyond version 60 is less certain, given the extreme rate of change in Firefox platform code. We will work with add-on authors in the coming months to determine a viable add-ons strategy.