Normally, Duecourse tries to detect unsupported versions via feature checking rather than UA sniffing.
A recent issue with the composer in older versions of Firefox required them to use UA sniffing because if you asked certain versions of Firefox they would happily (and technically accurately) say they supported the necessary functions when they are in fact buggy and don’t work properly.
The issue here is that Seamonkey is telling sites by default that it is compatible with a much older version of Firefox rather than the version it is actually compatible with.
Discourse is doing the right thing in this case.