If you have AVG software then see:
AVG Antivirus and Security Software - AVG Security Toolbar Help New Tab Search
http://www.avg.com/special-toolbar-how-to-disable-search-tlbrf.tpl-mcr1\

See also http://kb.mozillazine.org/Preferences_not_saved

Your above posted system details show outdated plugin(s) with known security and stability risks.

• Java Plug-in 1.6.0_12 for Netscape Navigator (DLL Helper)

Update the Java plugin to the latest version.

• http://java.sun.com/javase/downloads/index.jsp (Java Platform: Download JRE)

My antivirus is ANTIVIR.

I don't find any prefs.js on the disk.

Excuse me; I find the file and I try to use about:config for changing keword.URL to http://www.google.fr/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=

When I stop firefox and restart it

web search comming back to Yahoo.