Hi, it's likely that you are 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.

No it did not work, restart Firefox and get the same annoying Firefox has blocked an unsigned add-on even when using the supposed "signed" add-on link above, i had previously tired the "approved link" several time as well as just re-installing it

Hi, please make sure that you remove the unsigned version before installing the new one. Add-ons Manager (Ctrl+Shift+A).

Removed the "unsigned version" although every time I install it it says it is signed. the tool bar installs then to test it I close and restart Firefox and as usual I get the not allowed message and have to reinstall the toolbar

Running Firefox 43.01 and it says I am up to date

Really getting frustrated very close to uninstalling and switching to a browser that works

Hi again, sorry you are having so much trouble with this. It now seems that this is a bug, and the engineering team is currently working with the folks at Yahoo to get this fixed.

Thank you

Followup: The Yahoo toolbar add-on was updated on December 29, 2015. The latest version of the Yahoo toolbar fixes an issue where the toolbar could not be verified for use in Firefox, which caused it to become disabled in Firefox 43.