Firefox has disabled all addons that are not signed for security reasons. To enable the addon again, you will have to allow Firefox to enable addons that are not signed.

You can enable the addon signing by doing the following:

1. Type about:config into the address bar
2. Click the understand button
3. Locate the xpinstall.signatures.required boolean
4. Change the value to false
5. Go to the addon manager
6. Enable all desired addons

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

You can refer to this Mozilla support article for further information:

• Add-on signing in Firefox

Warning: Allowing addons to be installed if they are not signed may create a security risk. Use with caution and at your own risk.

Alternatively, try the solution given by philipp in https://support.mozilla.org/en-US/questions/1099042#answer-817786

hi, please try the version from https://addons.mozilla.org/firefox/addon/yahoo-toolbar/

Thank you!  :-)

Yahoo Toolbar is signed but it appears to be behaving in a way that invalidates its signature. The Mozilla Addon team will loop in Yahoo to help diagnose this issue. Hopefully this will be resolved quickly.

In the mean time you could use Wesley's solution keeping his warning in mind.

Thanks

I did what Wesley said & everything seems to be OK now (for the time being on this issue).

Thank you!

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 for the Yahoo toolbar problem, you should reset the preference back to true (the default value).