1. First of all, you need to download the package for the version you want to install. You can get it from anywhere on the internet. I found out this official location for all mozilla releases http://releases.mozilla.org/pub/mozilla.org/firefox/releases/
2. Download the version that you need and that supports the architecture of our current FAI of Ubuntu 11.04-32 bit.
3. I tried this for Firefox-3.6. What you get from this link is a tar.bz2 file which needs to be extracted. Extract it and you will get a file named firefox in this extracted folder.This firefox file is a self executable file, just double click and you are ready to go with firefox version you want.
4. To make things easier for the users, create a launcher on the desktop and convert it into an icon. Here is a screenshot: