Is it possible to downgrade my iPod touch firmware (from 4.2 to 4.0) from Ubuntu?
I believe it's possible but it's hard. There's a tutorial on how to restore iPod firmware on the rockbox site: http://www.rockbox.org/wiki/IpodManualRestore
Yes, its possible, but it involves a lot of work manually converting firmware images and messing with boot files.
You will also have to download a few tools that will allow you to read and write to the iPod's boot sectors.
Basically, if it's at all possible use the iTunes method, unless you want to learn how stuff like firmware updates work (the hard way).
Apologies for the lack of supporting links, but this will involve plenty of Googling regardless....
You should check libimobiledevice. There is no GUI as of yet, but they're working on getting ipod touch models to work on linux (including software up and downgrade) http://www.libimobiledevice.org/