Fixed!
Back in feb 2004, I had written about what was the most insane bugfix ever, in my opinion.
Bug 150099 had changed the behavior of close-tab to hide the last open tab, instead of closing it. Bug 156082 was reported to have the insanity reverted, but never got fixed. Someone even came up with an extension to replace the last open tab with about:blank when closed.
Okay, enough of history. This was fixed for firefox via bug 248987. Thanks to mconnor for checking it in.
In short: firefox rocks, seamonkey sucks!
;-)