Application that crashed: kmail
Version of the application: 1.12.3
KDE Version: 4.3.3 (KDE 4.3.3)
Qt Version: 4.5.2
Operating System: Linux 2.6.32-020632rc6-generic i686
Distribution: Ubuntu 9.10

What I was doing when the application crashed:
I tried to open a link in an email. Then, kmail opened a dialog and asked me to choose an application for opening the link. I choosed firefox from the menu and "firefox %u" appeared in the text field. After clicking the button to proceed, firefox showed up and kmail segfaults.

I'd like to mention the link in the mail was set like:
(Name of site, http://www.foo.bar/address/extension).
Firefox shows in its address bar:  
http://www.foo.bar/address/extension).
Thus, the site didn't show up because of ")." at the end.

I was able to reproduce the crash several times.

 -- Backtrace:
Application: KMail (kmail), signal: Segmentation fault
[KCrash Handler]
#6  0xb4a8a09d in KIO::Slave::deref() () from /usr/lib/libkio.so.5
#7  0xb4a8ac46 in KIO::Slave::gotInput() () from /usr/lib/libkio.so.5
#8  0xb4a8d033 in KIO::Slave::qt_metacall(QMetaObject::Call, int, void**) () from /usr/lib/libkio.so.5
#9  0xb5dfe263 in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/libQtCore.so.4
#10 0xb5dfeec2 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () from /usr/lib/libQtCore.so.4
#11 0xb4995057 in KIO::Connection::readyRead() () from /usr/lib/libkio.so.5
#12 0xb4996e1e in ?? () from /usr/lib/libkio.so.5
#13 0xb4996f4e in KIO::Connection::qt_metacall(QMetaObject::Call, int, void**) () from /usr/lib/libkio.so.5
#14 0xb5df6f0b in QMetaCallEvent::placeMetaCall(QObject*) () from /usr/lib/libQtCore.so.4
#15 0xb5df85fe in QObject::event(QEvent*) () from /usr/lib/libQtCore.so.4
#16 0xb5ffcf54 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQtGui.so.4
#17 0xb600467c in QApplication::notify(QObject*, QEvent*) () from /usr/lib/libQtGui.so.4
#18 0xb755814a in KApplication::notify(QObject*, QEvent*) () from /usr/lib/libkdeui.so.5
#19 0xb5de86cb in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/libQtCore.so.4
#20 0xb5de92b2 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/libQtCore.so.4
#21 0xb5de947d in QCoreApplication::sendPostedEvents(QObject*, int) () from /usr/lib/libQtCore.so.4
#22 0xb5e133ff in ?? () from /usr/lib/libQtCore.so.4
#23 0xb3d8ae78 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
#24 0xb3d8e720 in ?? () from /lib/libglib-2.0.so.0
#25 0xb3d8e853 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#26 0xb5e1302c in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#27 0xb609dbe5 in ?? () from /usr/lib/libQtGui.so.4
#28 0xb5de6c79 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#29 0xb5de70ca in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#30 0xb5de953f in QCoreApplication::exec() () from /usr/lib/libQtCore.so.4
#31 0xb5ffcdd7 in QApplication::exec() () from /usr/lib/libQtGui.so.4
#32 0x0804a702 in _start ()

Reported using DrKonqi