Since apt-get and yum won't let you install multiple versions of firefox I will explain how to here.

1. Go to this page and decide which version of firefox you want.
I used this one.

2. Unzip the archive
tar -zxvf firefox-9.0.1.tar.bz2
-z means filter the archive through gzip
-x extract
-v verbose
-f file name

3. Put firefox in its own directory.
mv firefox firefox-9.0.1
4. Make a backup of your profile direcory
cp -r /home/bob/.mozilla /home/bob/.mozilla3
If you use seamonkey or thunderbird there profile info will also be there.

4. Create a new profile so you don't corrupt your old profile.
firefox -CreateProfile "firefox-9.0.1 /home/bob/.mozilla/firefox/firefox-9.0.1"
This is my profile name firefox-9.0.1 and this is my profile directory /home/bob/.mozilla/firefox/firefox-9.0.1

5. Run firefox 9.0.1 from the directory I put it in.
/home/bob/firefox-9.0.1/firefox -no-remote -P firefox-9.0.1
/home/bob/firefox-9.0.1/firefox is my install directory
-no-remote allows to use firefox 9.0.1 and my old version of firefox
-P is the profile option
firefox-9.0.1 is the name of my profile

These 2 links go into more depth if you are interested.

Don't do this like one of the links say to do.
firefox -no-remote -CreateProfile firefox-3.6
That creates a profile called firefox-3.6 like you would expect but it puts profile in the firefox directory instead of creating its own separate directory. So in this case it would be in /home/bob/.mozilla/firefox/ instead of /home/bob/.mozilla/firefox/firefox-3.6 like you would expect.