AMO Browsing for SeaMonkey
When this add-on has been installed it will add a "View Version History" link to the download page of Firefox or Thunderbird add-ons. This link takes you to a new page, which lists the most recent versions of an add-on, and offers install, convert, and/or download buttons when appropriate.
Usually, you will want to use the most recent version of an extension that has SeaMonkey support, or (if there is none) the most recent version that does not require WebExtensions.
Add-ons that use WebExtensions are not compatible with SeaMonkey, but you might be able to find older, non-WebExtensions versions that will work.
Download from AMO: https://addons.mozilla.org/en-US/seamonkey/addon/amo-browsing-for-seamonkey/
mozillaZine thread: http://forums.mozillazine.org/viewtopic.php?f=40&t=2834855