I suspect that an add-on is causing this error. Try the procedures in the troubleshooting extensions and themes article.

OK problem solved. It seems the 2 problems I was having were related and both were fixed by uninstalling the "Fast Browser Search" extension. This was done by going to Tools->Add Ons->Extensions.

You may also need to reset the pref keyword.URL and the user agent that have been changed by Fast Browser Search (FBSMTWB)

general.useragent.override:

Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10.4; en-US; rv:1.9.2.10) Gecko/20100914 Firefox/3.6.10 FBSMTWB

keyword.URL:

http://www.fastbrowsersearch.com/results/results.aspx?s=NAUS&v=19&tid={E67318B9-71E7-FBB0-FC7E-7CF8756187B3}&q=

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

You reset a pref via the right-click context menu.
See http://kb.mozillazine.org/about%3Aconfig