Ubuntu is a "flavour" of Linux. "Gnome" is a desktop environment/window manager, and is responsible for looking after the windows and stuff, but doesn't work terribly well in 11.10 anymore
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I upgraded to Ubuntu 11.10 recently and am forced into using Unity until I see how some of the other desktop environments behave. Really don't like the way Unity handles the window bars and the side bar can go die in a fire for all I care for it
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Installing plugins for Firefox and Chromium shouldn't be too hard. Usually it's a case of clickin' buttons and following instructions.
Never used autologin (I want a password on my machine), so can't really advise on that.
If you're using Unity (the new default window manager) click the "cog + spanner" icon (system settings) go to "Screen" then turn off "Lock", so you don't need to type in your password after you've left your machine alone.
On the files thing - if you have a Windows install on your machine, under the file manager, you should have a "SomethingGB File System" that should be your windows drive.
By default, files under Linux save to the "home" folder, which is at /home/yourusername. Also typing "~" in the file manger's location box or terminal will go to there too.
