To find the version (from terminal): cat /etc/lsb-release

You can also find out from Firefox by going to Help, About Mozilla Firefox. :)

Another useful thing to know is the kernel version. Use (from terminal): uname
-r

You can upgrade from the previous version. You cannot skip versions, so must
upgrade to the previous version before upgrading to the latest. So the upgrade
path from Hardy (8.04) would be to Intrepid (8.10), then Jaunty (9.04). The
exception to this is from one LTS to the next which would be from Dapper 6.06 to
Hardy 8.04 to something that begins with L 10.04.

Yes the upgrade manager will give you the option to upgrade. At the top it will
say there is a new version with an upgrade button.