I remember running into the same problem. For some unknown reason, they did not post any install instructions. Since the tar files are compiled binaries, I followed the instructions for an earlier release. Here is a sample.
Extract the tarball in the directory where you want to install Firefox:
tar -xzvf firefox-1.5.tar.gz
This will create a firefox subdirectory of that directory.
Change the above to match the tar file you downloaded.
Things to consider, they do recommend deleting your old install. It is up to you, I did, and I did not loose any bookmarks. I believe they are stored in your /home/user directory.
Copy the tar file to the area of the file system you want to install to. I put mine in /usr and installed as root. If you install it to your home directory, only your user can access firefox. That is O.K. if it is a single suer system. I have several user, so I put it in /usr.
Running the tar command there will create a directory structure. Last thing I did was create a desktop icon to launch. It will launch form the command line also, but you will have to specify the path to firefox.
The other thing I forget, is changing the symlink to java. If you don't, you will get messages about plugins not being installed.
Add what ever add-ons you like. The ones for older versions probably won't work.
That is about it.