due to the fact that BT4 pre-final is based on Intrepid, it's quite a challenge to install FF 3.5 as it's not at all available via apt-get. There's a rather comfortable python-script called "Ubuntuzilla" available though that automates the process by downloading FF 3.5 directly from Mozilla, put FF 3.0 aside and is even able to revert the whole process with a single command. Furthermore "Ubuntuzilla" introduces a cron job that notifies you of updates on FF 3.5. So, let's get started:

1st Step: Download the "Ubuntuzilla" installer from Sourceforge and install it utilizing dpkg and apt-get to resolve dependencies (ignore the warnings while running dpkg)
# wget
# dpkg -i ubuntuzilla-4.6.1-0ubuntu1-i386.deb
# apt-get install -f
2nd Step: Execute the "Ubuntuzilla" python-script and follow the rather simple y/n questions (should be self-explanatory)
# -a install -p firefox
"Ubuntuzilla" puts FF 3.0 (installed from repositories) aside utilizing dpkg-divert and replaces the KDE menu-item to launch FF 3.5 instead of FF3.0 (both are installed in parallel). FF 3.5 gets btw installed to /opt/firefox

If you want to revert the process for some reason, you just need to execute the "Ubuntuzilla" python-script again like this
# -a remove -p firefox
Have fun! For an even smoother FF experience, please also checkout my other tutorial about "Integrating GTK-apps into the KDE3 desktop environment" here