It's really not that tricky but there was one minor gotcha in that
firefox was assuming the wrong libstdc++ and to fix that required
using Synaptic and select whatever other libstc++ wasn't installed;
I think it was version 5.

But here's the procedure.

First get the latest version of firefox, presently 2.0.0.18.

Visit this URL and grab the language version you want:

<www.mozilla.com/en-US/firefox/all-older.html>

Visit here if you also want the latest version of Thunderbird,
noting the latest in the Ubuntu repositories is the obsolete 2.0.0.17
version:

<www.mozilla.com/.../all-older.html>

The following steps must be done as root; "sudo bash" will work.

If you don't already have one, create a /usr/local directory, then
create a /usr/local/mozilla directory.

Copy the thunderbird-2.0.0.18.tar.gz file to /usr/local/mozilla

Unpack the tar file per:

cd /usr/local/mozilla
zcat thunderbird-2.0.0.18.tar.gz | tar xvof -

That's it for steps needing to be performed as root.

A "/usr/local/mozilla/firefox" must be added to your PATH. Best to
do it in the bash setup, but for a quick test this works:

export PATH=/usr/local/mozilla/firefox:$PATH

Now simply start firefox from the command line:

firefox &

If you receive an error about a missing libstdc++, then you'll have
to use Synaptic, search for "libstdc", and install whatever version
you don't already have. That's it.

Plugins may or may not have to be manually installed; I really don't
use any, but installation instructions "should" accompany plugins.

The only other matter is the firefox icon for the desktop.

Create a launcher for /usr/local/mozilla/firefox/firefox
Click the default icon in the upper left portion of the launcher
creation window to browse, select "Filesystem", then navigate to
/usr/local/mozilla/firefox/icons and click "Open". The one that
works is mozicon128.png

It's nice not having to suffer FF3's bugs!