There are a plethora of install instructions out there for Firefox 3.6 Karmic (which uses Firefox 3.5 by default), however it mostly suggests the mozilla-daily build.
By chance, I found the firefox-stable ppa (https://launchpad.net/~mozillateam/+archive/firefox-stable/)
Here’s how (enter each in turn):
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
Here’s what it looks like:
~ $ sudo apt-get upgrade
Reading package lists… Done
Building dependency tree
Reading state information… Done
The following packages have been kept back:
firefox firefox-3.5 firefox-3.5-branding firefox-gnome-support
0 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.
~ $ sudo apt-get dist-upgrade
Reading package lists… Done
Building dependency tree
Reading state information… Done
Calculating upgrade… Done
The following NEW packages will be installed:
firefox-branding
The following packages will be upgraded:
firefox firefox-3.5 firefox-3.5-branding firefox-gnome-support
4 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 12.5MB of archives.
After this operation, 32.4MB of additional disk space will be used.
Do you want to continue [Y/n]? y
You will now get /usr/binfirefox as Firefox 3.6