Changes

Mozilla core have disabled "legacy" add-ons (add-ons which are not based purely on WebExtensions) in Firefox 57. Because this type of legacy add-on is ons are not supported from mozilla57 any more, many interfaces have been removed from Mozilla core.

HoweverDespite the interface removals, Thunderbird 57-60 and beyond and Seamonkey will '''<u>still support''' </u> these add-ons <u>if </u> the appropriate modifications are done to the add-on to adapt to the changed interfaces'''.  In generaladdition, Thunderbird is will be adding WebExtension support ({{bug|1396172}}) while maintaining "legacy" add-ons and hybrid add-ons.<br><p>

# If addonadd-on's install.rdf contains no <em:optionsType> declaration, the addonadd-on's preferences page (<em:optionsURL>) will only be accessible via a new submenu in Tools menu (implemented in {{Bug|1419145}}, landed on Daily 59 as of 2017-12-22). The preferences will open in a dialog, as was the case in older versions. [[File:Add-on Options.png|none|Screenshot showing Add-on Options in TB 59]]# If addonadd-on's install.rdf contains <em:optionsType>3</em:optionsType>, the Preferences button will appear in the Add-on Manager tab besides the addonadd-on, as was the case in older versions. Clicking the button will open the preferences page in a new tab. The preferences page (optionsURL) is also accessible via the new submenu in Tools menu and will open in a tab. No other value of <em:optionsType> is supported and TB will even refuse to install such addons add-ons (decision from {{bug|1414406}}).