IIUC, extensions for SeaMonkey are not at addons.mozilla.org any longer but at
https://addons.thunderbird.net/
Classical extensions (not WebExtensions) mostly work with SeaMonkey 2.53.x and lower. The latest "official" such release is 2.53.5.1, available from
https://www.seamonkey-project.org/releases/ and also published by openSUSE Linux on their Update-OSS repository for openSUSE Leap 15.2 (I don't know about other distros or OSes). SeaMonkey 2.53.6b1pre nightlies are avilable
unofficially for Linux32, Linux64, Win32, Win64 and Mac in .en-US and .de localizations from
http://www.wg9s.com/comm-253/ I use that, and I have a lot of extensions, many of which give me no problem; but use it at your own risk, and take a backup of your profile before you try if you want to be able to go back to whatever you used before.