Hi, we appreciate that it's very frustrating, but it's likely that you were using an unsigned version which is no longer supported, so please try this signed Yahoo Toolbar add-on instead.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

The Yahoo toolbar add-on becoming disabled in Firefox is a known issue that Mozilla and Yahoo are working on. The problem is related to the new add-on signing requirements that became mandatory in Firefox 43.

In this particular case, you can follow the instructions in the Add-on signing in Firefox article under Override add-on signing (advanced users): Go to the Firefox Configuration Editor (about:config page) and double-click on the xpinstall.signatures.required preference to toggle the setting to false. Important: Changing this preference is only a temporary solution which will no longer work in Firefox 44.

Followup: The Yahoo toolbar add-on has been updated. The latest version fixes the problem. If you have set the xpinstall.signatures.required preference to false in the Firefox Configuration Editor (about:config page) as a workaround, you can go back now and reset the preference to true.