What's your computer system and Firefox version?

Is Yahoo your default search engine? No did you want to use something else, but Yahoo comes in instead.

Is it just the search that is slow? What about the rest of the time.

Considering that Yahoo!Search was removed from Firefox quite awhile ago, chances are you have picked up some Malware that has snagged your search engines in Firefox.

See these Knowledge Base articles for help: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware https://support.mozilla.org/en-US/kb/searches-take-you-to-wrong-search-website

I use speed dial for background thats probably it uses yahoo