How have you reset it? Did you reset the keyword.URL preference as shown in the location bar search article?

If you did that and it still changes back to Yahoo, follow the instructions in the preferences are not saved article. Also check any add-ons/toolbars that you have installed to see if one of them has a setting that is changing it to Yahoo.

Your above posted system details show outdated plugin(s) with known security and stability risks.

1. Adobe Shockwave for Director Netscape plug-in, version 11.0
2. Shockwave Flash 10.1 r53

Update the Flash and Shockwave for Director plugin to the latest version.

• http://www.adobe.com/software/flash/about/
• http://www.adobe.com/shockwave/welcome/

I finally figured out how to remove it on my end.

1. about:config filter area...type yahoo

Double click on anything that shows as yahoo search...change to none (on my end there were two...one on top of the other.

2. In the URL bar, type in "about:config"

   In the filter bar type "defaulturl"
   Change the value there to take whatever search engine with whichever   parameters you want.  

Good luck!