Hi,
There has been many features added and bugs fixed. The easiest way for you to find out is to have a look at the release notes, which can be found here
http://www.juniper.net/techpubs/software/screenos/
Have a look at the release notes for 5.0, 5.1, 5.2, 5.3 and 5.4 to see the features over time.
As for upgrading the firewall you cant get it to download the software directly from Juniper. You need a support contract so that you can login and download it to you PC and then you can upload it to the firewall, either via TFTP or through the web browser.
Regards
Andy