Does this happen in the middle of a session? You may have an unwanted extension that is modfying search behavior. Try this:

Disable ALL nonessential or unrecognized extensions on the Add-ons page. Either:

• Command+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable.

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

If the change occurs only between sessions, when you exit and start up Firefox again, check for a user.js file. This article describes how to do that: How to fix preferences that won't save.

Thanks. I had already disabled all extensions although there was not one for Yahoo. This event just happened again. I clicked on a link and it went immediately to a yahoo search page. I thought that by managing my search engines and getting rid of Yahoo would solve the problem but apparently it does not. Also, in my preferences under Certificates and View Certificates, I find certificate names for Yahoo. I keep deleting them but they only return.

Could you test something: try doing two-word search in the address bar to confirm that you get Google results (or your preferred search engine) in that scenario. Then try a one-word search (no spaces) and see whether you get different results.

(Firefox will check whether one word searches are valid server names (e.g., intranet, or example.com) and if they are not, some service providers, instead of returning that status code, will display their own page of search results. Just wondering whether this might explain how Yahoo is getting into the picture.)

If you have avg web tune up installed it will constantly rewrite your Keyword.url to YAHOO! because they sponsor avg and that is who avg uses for "safe secure search". Just go to programs and features click on the web tune up and uninstall. It will ask you if you want to keep the secure web search or revert back to your browsers default. Click the option to revert and then click uninstall. You are good to go at this point.