You do not need to keep older versions unless you prefer to.
They may be safely deleted without interfering with the current
version of TenFourFox.
Unfortunately, do keep in mind that as web pages become more
complex so does the browser, and sadly our venerable machines will
never get any quicker. We always recommend making sure you remove
add-ons you don't need, install as much RAM as your computer will
handle (on a G5 4GB would be a very reasonable amount), and
consider useful upgrades within your budget such as switching your
hard disks for SSDs.