hello crash81, the newest version of firefox is firefox 18. please note that using outdated versions of firefox [https://www.mozilla.org/security/known-vulnerabilities/firefox.html affected by known security vulnerabilities] will put the integrity of your system and all the personal data on it at risk! so in my opinion it's not a good idea to downgrade just for the sake of an addon. in case you're having issues with the yahoo toolbar please first make sure that you've installed the latest version available or else contact yahoo's support - they can likely give you more detailed guidance and are the only ones who can fix bugs or make necessary adjustments in the addon & i suppose it's also very much in their interest for the toolbar to work in current browser versions. thank you