Hi szaemon,
Chin up - we will eventually get through all this.
I wonder if some of my trouble could be caused by the fact that I'm using an Alpha version of Ubuntu.
Possible? Yes.
Probable? I have my doubts.
As Cell has alluded to in an earlier post, I too have seen numerous references to your particular ethernet issue being reported as a bug.
My understanding is that if you can access the internet through your modem via Windows, then your modem is not the issue. By trying to type in an ip address directly in Firefox to access the modem in Linux and not having success tells me its the Ethernet port under Linux.
The suggestion of Ubuntu 9.10 Alpha was to see if the upcoming release would fix your video, and it has. This means that when Ultimate Edition 2.4 is released, your video issue will be resolved. But it has also shown us that you may have another issue, namely internet access through your on board ethernet.
I'm certainly no Ethernet/Linux Guru - far from it. If you wish to try and get some help from the Ubuntu forums, by all means, it's a great idea - I would suggest before you do, you download Ubuntu 9.10 Beta - which has superseeded Alpha.
We will of course still keep trying to find a solution for you, because it will also be an Ultimate Edition 2.4 issue. Either way, don't give up. Linux is always a learning experience.
I hate to suggest this, but if you are really really desperate, a Wireless USB network dongle and a wireless router attached to your modem could bypass this issue altogether. Or maybe even one of those USB to Ethernet conversion cables to convert one of your USB ports into another Ethernet port... just thinking outside the box here.