Ok definitively, absolutely unquestionably a bug. Or a Gnome fan conspiracy. Or maybe an unholy alliance between the Gnome fans and the Konqueror developers...

Look, fresh untampered, updated install of Kubuntu 9.04.

sudo apt-get install firefox
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  firefox-3.0 firefox-3.0-branding libidl0 libstartup-notification0
  xulrunner-1.9
Suggested packages:
  ubufox firefox-3.0-gnome-support latex-xft-fonts
The following NEW packages will be installed
  firefox firefox-3.0 firefox-3.0-branding libidl0 libstartup-notification0
  xulrunner-1.9
0 upgraded, 6 newly installed, 0 to remove and 0 not upgraded.
Need to get 8804kB of archives.
After this operation, 25.2MB of additional disk space will be used.

sudo apt-get install firefox-3.5
Reading package lists... Done                                                  
Building dependency tree                                                       
Reading state information... Done                                              
The following extra packages will be installed:                                
  apturl docbook-xml esound-clients esound-common firefox firefox-3.0          
  firefox-3.0-branding firefox-3.5-branding gamin gconf2 gconf2-common gksu    
  gnome-app-install gnome-icon-theme gnome-keyring gnome-mime-data gnome-mount 
  gvfs gvfs-backends libaudiofile0 libavahi-glib1 libbonobo2-0                 
  libbonobo2-common libbonoboui2-0 libbonoboui2-common libcairo-perl           
  libcanberra0 libcdio-cdda0 libcdio-paranoia0 libcdio7 libcroco3 libesd-alsa0 
  libgail-common libgail18 libgamin0 libgconf2-4 libgcr0 libgksu2-0            
  libglade2-0 libglib-perl libgnome-keyring0 libgnome2-0 libgnome2-canvas-perl 
  libgnome2-common libgnome2-perl libgnome2-vfs-perl libgnomecanvas2-0         
  libgnomecanvas2-common libgnomeui-0 libgnomeui-common libgnomevfs2-0         
  libgnomevfs2-common libgnomevfs2-extra libgp11-0 libgsf-1-114                
  libgsf-1-common libgtk2-perl libgtkhtml2-0 libgtop2-7 libgtop2-common        
  libgvfscommon0 libidl0 liblaunchpad-integration1 libnotify1 liborbit2        
  libpam-gnome-keyring libpolkit-gnome0 libproxy0 librsvg2-2 librsvg2-common   
  libscrollkeeper0 libsexy2 libsoup-gnome2.4-1 libsoup2.4-1                    
  libstartup-notification0 libtdb1 libvte-common libvte9 libwnck-common        
  libwnck22 libxres1 notification-daemon policykit-gnome python-cairo          
  python-gconf python-glade2 python-gst0.10 python-gtk2 python-gtkhtml2        
  python-launchpad-integration python-pyorbit python-sexy python-vte
  scrollkeeper sgml-data software-properties-gtk synaptic ubufox xulrunner-1.9
  xulrunner-1.9.1
Suggested packages:
  docbook docbook-dsssl docbook-xsl docbook-defguide firefox-3.0-gnome-support
  latex-xft-fonts firefox-3.5-gnome-support cryptsetup libbonobo2-bin
  libcanberra-gtk0 esound desktop-base libgnomevfs2-bin libgtk2-perl-doc
  librsvg2-bin python-gconf-dbg python-gtk2-doc python-gst0.10-dbg
  python-numpy python-gnome2-extras-doc python-launchpad-integration-dbg
  python-pyorbit-dbg perlsgml doc-html-w3 opensp dwww menu deborphan
The following NEW packages will be installed
  apturl docbook-xml esound-clients esound-common firefox firefox-3.0
  firefox-3.0-branding firefox-3.5 firefox-3.5-branding gamin gconf2
  gconf2-common gksu gnome-app-install gnome-icon-theme gnome-keyring
  gnome-mime-data gnome-mount gvfs gvfs-backends libaudiofile0 libavahi-glib1
  libbonobo2-0 libbonobo2-common libbonoboui2-0 libbonoboui2-common
  libcairo-perl libcanberra0 libcdio-cdda0 libcdio-paranoia0 libcdio7
  libcroco3 libesd-alsa0 libgail-common libgail18 libgamin0 libgconf2-4
  libgcr0 libgksu2-0 libglade2-0 libglib-perl libgnome-keyring0 libgnome2-0
  libgnome2-canvas-perl libgnome2-common libgnome2-perl libgnome2-vfs-perl
  libgnomecanvas2-0 libgnomecanvas2-common libgnomeui-0 libgnomeui-common
  libgnomevfs2-0 libgnomevfs2-common libgnomevfs2-extra libgp11-0 libgsf-1-114
  libgsf-1-common libgtk2-perl libgtkhtml2-0 libgtop2-7 libgtop2-common
  libgvfscommon0 libidl0 liblaunchpad-integration1 libnotify1 liborbit2
  libpam-gnome-keyring libpolkit-gnome0 libproxy0 librsvg2-2 librsvg2-common
  libscrollkeeper0 libsexy2 libsoup-gnome2.4-1 libsoup2.4-1
  libstartup-notification0 libtdb1 libvte-common libvte9 libwnck-common
  libwnck22 libxres1 notification-daemon policykit-gnome python-cairo
  python-gconf python-glade2 python-gst0.10 python-gtk2 python-gtkhtml2
  python-launchpad-integration python-pyorbit python-sexy python-vte
  scrollkeeper sgml-data software-properties-gtk synaptic ubufox xulrunner-1.9
  xulrunner-1.9.1
0 upgraded, 101 newly installed, 0 to remove and 0 not upgraded.
Need to get 36.8MB of archives.
After this operation, 167MB of additional disk space will be used.

25MB disk space correct, 167MB disk space not correct.

Vanilla firefox 3.5 downloaded, 26MB space used when extracted. Vanilla FF3.5 works just fine but is annoyingly not going to be handled by apt. Therefore those additional libraries are *not* dependencies, therefore a bug.

For the future, consider this: change package manager default options in Gnome to always install "suggests" & "recommends" (if making everything all work the one way is what you want), let apt-get just load "depends" and let Kubuntu do the same as apt-get.

In the meantime, can the dependency be removed so Kubuntu users can install FF3.5 without 140MB of unnecessary files? Or information posted here on how to prevent it (some config file somewhere?). Please? Pretty please?