Changes

<big>'''Add-ons Guide for Thunderbird 57 and beyond (including version to Thunderbird 60)ESR'''</big>

(For Thunderbird 62 and beyond please visit the [https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_62 Add-ons Guide 62]. Thunderbird 61 never existed, so we're skipping that number.) '''Legacy add-ons''': Thunderbird 57-60 and Seamonkey will '''<u>still support</u> legacy add-ons <u>if</u> add-on author makes modifications to adapt to the changed interfaces'''. The items to be modified are listed below. (The reasons for these changes is Firefox platform have disabled "legacy" add-ons in Firefox 57, add-ons which are not based purely on [[WebExtensions]], so Mozilla removed core interfaces.) The picture beyond Thunderbird version 60 is less certain, given the continued high rate of change in Firefox platform code. We will work with add-on authors in the coming months to determine a viable post-60 add-on strategy.