I just did a upgrade to my Kubuntu 7.10 system, now it's on 8.04. This is a LTS, or long-term support release. See, Ubuntu comes out with a new version every six months or so. Generally, once the new version is out, the old version is no longer supported except for security updates. If you don't upgrade to the new version and another new version comes out in the meantime... you're stuck.
The LTS versions are different. They're supported for years (last one was 6.10, from Oct. '06), and are supported for upgrade through the next LTS release. Basically, you could upgrade to 8.04 from 7.10 or 6.10.
Anyway, I found it odd that the default Firefox they use in 8.04 is 3.0 beta 5. I mean... it's not even a release candidate. And this is a LTS version. I just found it very odd. I guess it has something to do with how Ubuntu always likes to have the newest of everything.