[Update] As of June 5, issue is fixed with latest round of updates.
Grrrrr…
The latest FileZilla (3.5.3) that installs with Kubuntu-12.04 crashes when using the FileZilla Site Manager. This FileZilla behavior is actually caused by a bug in oxygen-gtk2 (1.2.1) which is the latest version that comes with Kubuntu-12.04.
Reference:
http://trac.filezilla-project.org/ticket/7968
https://bugs.kde.org/show_bug.cgi?id=295875
Apparently this oxygen-gtk2 behavior occurs with versions 1.2.2 onwards but not with the 1.2.0 (older) version. It is fixed in oxygen-gtk2 1.2.3. but…. you’ll have to compile it (yuck).
Workarounds:
For me the oxygen-gtk2 bug only seems to affect FileZilla so I’m not that keen on compiling a version 1.2.3 just to get FileZilla to behave. So here are a 2 work-arounds.
1. Use the “Raleigh” GTK+ Style instead of the default oxygen-gtk (style). FileZilla works properly but looks fugly. Firefox will also look funky.
Go to: System Settings | Application Appearance | GTK+ Appearance
For the “Widget style”: Choose Raleigh
Select “Apply”.
Done.
2. In FileZilla, open a Quick Connection FIRST before tying to use the Site Manager. After a Quick Connection session is initiated the FileZilla + Site Manager behaves properly. Just make sure to ADD the second (Site Manager) session and not replace the first one (Establish a New Connection). Of course, you ‘ll need to keep the first (Quick Connect) session connected while you’re working on any sessions.
Since oxygen-gtk2 folks have already released a fixed version, I’ll just sit back a while and wait for the updated version.
Ciao.

Did the fixed version appeard?
Petre,
No, unfortunately not. Kubuntu-12.04 is still on 1.2.2
miguel@terra:~$ dpkg -s gtk2-engines-oxygen
Package: gtk2-engines-oxygen
Status: install ok installed
Multi-Arch: same
Priority: optional
Section: gnome
Installed-Size: 1406
Maintainer: Kubuntu Developers
Architecture: i386
Version: 1.2.2-0ubuntu1
Depends: libc6 (>= 2.4), libcairo2 (>= 1.10.0), libgcc1 (>= 1:4.1.1), libgdk-pixbuf2.0-0 (>= 2.22.0), libglib2.0-0 (>= 2.26.0), libpango1.0-0 (>= 1.18.0), libstdc++6 (>= 4.6), libx11-6
Pre-Depends: multiarch-support
Suggests: kde-config-gtk-style
Description: Oxygen widget theme for GTK+-based applications
Oxygen-Gtk is a port of the Oxygen widget style to GTK+.