how do I download ff3.5?

Let me rephrase that: How do I download AND install it?

My suggestion would be to wait for karmic, but if you really want to install firefox 3.5 look in the installing from source section.
FAQ #66: “Installing software in ubuntu”.

I just spent the last half hour or so frustratingly trying to do this.
In the end I found some helpful information over several questions.
So heres some help for newbies like me.

Using Ubuntu 9.04
I used the Synaptic package manager and searched for "Firefox-3.5" I got a whole lot of results and selected "firefox-3.5" I installed this along with the 3 suggested packages.
However since it did not replace Firefox 3.0 once installed I had no idea what to do to make it run.
Eventually I came across the answer that it was called "Shiretoko Web Browser" under applications/Internet.

I hope that helps someone

This is a 'universal' method valid for all Ubuntu releases

In short please open a Terminal from the menu Applications → Accessories → Terminal and type or copy and paste, one row a time then press enter:

echo 'deb http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu' $(lsb_release -cs)' main' | sudo tee -a /etc/apt/sources.list

echo 'deb-src http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu' $(lsb_release -cs)' main' | sudo tee -a /etc/apt/sources.list

give your user password when requested, you don't see nothing when you type it, then press enter

Then add this repository gpg key copy and paste this row into terminal and then press enter:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 247510BE

Then update your system type:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install firefox-3.5

Hope this helps

The method given by marcobra above is not what you want: it will install the current daily builds of the new firefox 3.6 (which might be very buggy)
I would recommend to use another repository:
https://edge.launchpad.net/~ubuntu-mozilla-security/+archive/ppa
(for jaunty)
deb http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu jaunty main
deb-src http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu jaunty main

using that repository you can install the 3.5 build of firefox.

I should add: installing firefox 3.5 became even easier because the final firefox 3.5 was updated on the security updates
If you are using jaunty (ubuntu 9.04): just go to applications add/remove and search for 'firefox-3.5' to install.

It looks like I should have mentioned I'm using 8.10. What difference will that make to these suggestions? I assume it's VERY obvious I'm in uncharted territory.

Let me be more explicit about this: For a fact I don't understand at least 95% of what I've been given, and I'm guessing I'll screw up trying to use the rest, if past history is any guide, but in any case this will really take some time, so I'll thank all of you in advance so you'll not think I've just blown you all off and disappeared.

Dan please follow the Johan instructions... to get stable release of Firefox-3.5
using https://edge.launchpad.net/~ubuntu-mozilla-security/+archive/ppa

In short please open a Terminal from the menu Applications → Accessories → Terminal and type or copy and paste, one row a time then press enter:

echo 'deb http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu' $(lsb_release -cs)' main' | sudo tee -a /etc/apt/sources.list

echo 'deb-src http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu' $(lsb_release -cs)' main' | sudo tee -a /etc/apt/sources.list

give your user password when requested, you don't see nothing when you type it, then press enter

Then add this repository gpg key copy and paste this row into terminal and then press enter:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A6DCF7707EBC211F

Then update your system type:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install firefox-3.5

Hope this helps

Please follow the instructions here and not in this answer:
https://help.ubuntu.com/community/FirefoxNewVersion