Seems there was some legacy settings causing this problem. Removing
~/.gconf/apps/nautilus solved it for me.
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nautilus is crashing and restart by gnome-session
https://bugs.launchpad.net/bugs/434921
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This seems to affect Thunderbird (and probably other XUL apps too) and
both Ubuntu packages and packages directly from Mozilla.org.
Both are working fine when using Human theme, but crash (usually from modal
windows) when using Pixbuf themes.
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gtk-qt-engine crashes Firefox with
