Here is what I did just some minutes ago:
I launched Gslapt > Update
Now I found the two versions of Firefox (ver. 3.0.8 and 3.6.13), then I right-clicked on 3.0.8 box and a dialog box appeared showing that I could remove it or upgrade it. So, first of all, I removed the newer one (ver. 3.6.13) then I upgraded the older one (ver. 3.0.

.
At last I launched Gslapt > Update again but firefox was not shown at all. However Firefox is present (ver. 3.6.15) and works all right.
Here is the gtkfind output for Firefox:
/home/ftp/pub/veclinux/packages/net/firefox-3.6.13-i586-1vl60.tlz
/home/ftp/pub/veclinux/packages/net/firefox-3.6.15-i586-1vl60.tlz
/home/alberto/.mozilla/firefox
/usr/bin/firefox
/usr/lib/firefox
/usr/lib/firefox-3.6.15
/usr/lib/firefox-3.6.15/firefox
/usr/lib/firefox-3.6.15/firefox-bin
/usr/lib/firefox-3.6.15/defaults/pref/firefox.js
/usr/lib/firefox-3.6.15/defaults/pref/firefox-branding.js
/usr/lib/firefox-3.6.15/defaults/pref/firefox-l10n.js
/usr/ports/contrib/firefox-djvu-plugin
/usr/ports/opt/firefox-acroread-plugin
/usr/ports/opt/firefox-flash-plugin
/usr/ports/opt/firefox-java-plugin
/usr/ports/opt/firefox
/usr/ports/opt/firefox/firefox-2.0-add-ldflags.patch
/usr/ports/opt/firefox/firefox-nopangoxft.patch
/usr/share/applications/mozilla-firefox.desktop
/usr/share/icons/ROX-noia/apps/firefox.png
/usr/share/icons/ROX-nuvola/apps/firefox.png
/usr/share/icons/Xquisite/scalable/apps/firefox-icon.png
/usr/share/icons/Xquisite/scalable/apps/firefox.png
/usr/share/icons/nuoveXT2/128x128/apps/firefox.png
........ lots of icons............... I removed from this list, just to shorten it
/usr/share/pixmaps/firefox.png
/var/log/packages/firefox-3.6.15-i586-1vl60
/var/log/scripts/firefox-3.6.15-i586-1vl60
/var/log/removed_packages/firefox-3.6.13-i586-1vl60
/var/log/removed_packages/firefox-3.0.8-i586-1vl60-upgraded-2011-03-07,18:18:27
/var/log/removed_scripts/firefox-3.6.13-i586-1vl60
/var/log/removed_scripts/firefox-3.0.8-i586-1vl60-upgraded-2011-03-07,18:18:27
By the way, is there anything I can delete?
Best Regards