Try this:

delete:

/etc/others-menu/*fennec.desktop
/opt/mozilla
/usr/bin/fennec
/usr/share/applications/hildon/fennec.desktop
/usr/share/dbus-1/services/org.mozilla.fennec.service
/usr/share/icons/hicolor/26x26/hildon/fennec.png
/usr/share/icons/hicolor/40x40/hildon/fennec.png
/usr/share/icons/hicolor/scalable/hildon/fennec.png

then (from postinst, maybe not needed)
update icons
# gtk-update-icon-cache -f /usr/share/icons/hicolor

update desktop
# update-desktop-database /usr/share/applications

In my case, I had the nightly version 9.0, it was not optimized for maemo 5, but the problem was solved.