Personally I'm an opera fan, so I haven't tried getting it to work but a quick search brought up: https://addons.mozilla.org/en-US/firefox/addon/51000, https://addons.mozilla.org/en-US/firefox/addon/12625 and https://addons.mozilla.org/en-US/firefox/addon/11551 as plug-is. As for blocking cookies there are several options.
I would have to look further into the workings of Google to understand that, but as I said: why should I invest the time? They shouldn't be forwarding me to German Google without my explicit desire to do so (which I would state by visiting google.de instead of google.com).
Also, I don't like to install plugins in general - who knows who made them and if they are safe. So why risk it? Using Yahoo! seems the cheaper course of action for now.