Addons that are not signed have been disabled in the latest version of Firefox for security purposes. To enable this addon again, please do the following:

1. Type about:config in the address bar
2. Click the I'll be careful button
3. Locate the xpinstall.signatures.required boolean
4. Change the value on this preference to false

Note: You may need to restart Firefox in order for the changes to take effect.

Warning: Doing the above steps may present a security threat. Please use with caution and at your own risk.

For more information about addon signing, please refer to this Mozilla support article.

• Add-on signing in Firefox

Followup: The Yahoo toolbar add-on was updated on December 29, 2015. The latest version fixes an issue where the Yahoo toolbar becomes disabled after restarting Firefox. If you changed the xpinstall.signatures.required preference to false in the Firefox Configuration Editor (about:config page) as a workaround, you should reset it back to true (the default value).