How about showing us what you're talking about??? What OS? What version of Firefox?

Here are some things to check:

(1) If your home page was changed to Yahoo -- you can test by pressing Ctrl+N to launch a new window -- check the steps in the following article: How to set the home page.

(2) If your new tab page was changed to Yahoo -- you can test by pressing Ctrl+T to open a new tab -- this may be due to one of your add-ons. This should be reported on the Settings page, just below the home page setting referenced in the article linked under #1.

(3) If your searches show up on Yahoo instead of Google, DuckDuckGo, or whatever you were using before, see whether you can change that back on the Settings page: Change your default search settings in Firefox. But if Firefox seems to be ignoring your setting, an add-on might be hijacking your searches over to a different site.

Firefox is a web browser and not a homepage nor a search engine.

Perhaps Yahoo .com has been set as a homepage in Firefox? https://support.mozilla.org/en-US/kb/how-to-set-the-home-page https://support.mozilla.org/en-US/kb/extension-changed-my-new-tab-page

The Firefox web browser (for desktop) has not come with Yahoo as one of the search engine defaults since Firefox 57.0 so it did not come in a Firefox install or update. /forums/contributors/712925