SeaMonkey 2.53 identifies itself as, Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 SeaMonkey/2.53.2
https://secure.informaction.com/update/?v=11.0.11rc1 looks as if it keys in on ""gecko": { "strict_min_version": "59" }", & presumably it is reading "60", & so it "just happens".
(SeaMonkey 2.53 isn't really "60", more "56", but has some ? backports ? whatever, courtesy of later FF versions, & identifies itself, spoof'd as it is, as "60".)
So unknowing user, errant click & oops.
Just how does that even happen to happen?
No install.rdf, so... ?
If you change to "strict_min_version": "61.0" in manifest.json, then it will not install.
So I guess this is really an issue on the SeaMonkey end, that must have some webextension parts in it (like FF 56), even if SeaMonkey does not support webextensions (other then it sure looks like they happen to install).
Ha! And you can even install from AMO, if you spoof your UA.