Comment 8 for bug 259387

As I described in comment 0 this bug is reported explicitly for Thunderbird and not SeaMonkey while this is covered in bug 307672. MsgComposeCommands.js is forked so will get a different behavior in SeaMonkey.

http://landfill.mozilla.org/mxr-test/mozilla/find?string=MsgComposeCommands.js

If you like to create a new bug for SeaMonkey feel free but I don't think its good to watch both products at the same time. The results are too different.