It seems it is easier if you just install this extension: http://https://addons.mozilla.org/firefox/3698/. Oh well, thanks to everyone who used what I made.
Quote from mellowmut
Np, it was only a little effort. 
That submit page doesn't want to accept my plugin. I made mine manually copying from already existing files on my searchplugin folder so it is missing some tags that page is looking for. The firefox extension link I gave produces the same result to the one I made so those tags must be unnecessary so I'm not gonna bother adding it to that page. Just install the extension it is more convenient and you can use it to other pages with a search form. 
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