when I type the name of a website in the URL barr I get a page like this

http://search.yahoo.com/search?p=graspop&fr=greentree_ff1&ei=utf-8&type=971163

when I go to about:config

keyword.URL is http://www.google.com/search?ie=UTF-8&sourceid=navclient&oe=UTF-8&gfns=1&q=

can someone tel me what the problem is and how I can use google I am feeling lucky instead of yahoo

Do you use a toolbar/extension that modifies the setting? Try closing Firefox and launching the browser in "safe mode" (you'll find the option in the Start Menu).

There is an AVG toolbar
It has a search feeld which uses yahoo
Maby that causes the problem

but... can I switch of that thing without loosing the AVG antivirus security

The avg addon – by my knowledge – only changes search engine result pages, thus you can switch off that addon without any consequences, as far as I know.

strange also that didn't help