You are sure did not install something did not want to ? Check add/remove programs and see what it says for Mozilla Firefox if it has Yahoo in the name and look for yahoo toolbars.

Also : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Use AdwCleaner also. Save the reports as should Google each result before deleting. If need help : https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/

Please let us know if this solved your issue or if need further assistance.

Normally Firefox starts up with a built-in home page and in some countries the default search provider is Yahoo. To check on or change those settings:

• How to set the home page
• Manage Firefox search settings

If the changes do not take or do not stick, then a bad extension (see Pkshadow's suggestions) or an issue mentioned in this article might be relevant: How to fix preferences that won't save.