You can download older versions of Firefox from this page. If you need a particular version and you can’t find it there, you may try constructing the download URL yourself. For example:
Will download Firefox version 3.5.9 for Linux in English. You may modify these three parameters to get any other version.
Unpackage the tarball and run firefox:
$ tar jxpvf firefox-3.5.9.tar.bz2
If you get the following error when executing firefox:
./run-mozilla.sh: 399: ./firefox-bin: not found
Then you probably need to install some libraries to support running 32 bits applications:
$ sudo apt-get install ia32-libs