1. Check to see if "Yahoo Application State Plugin version 1.0.0.7" is listed when you navigate to the Firefox Menu (three horizontal line button) > Add Ons (puzzle piece icon). Click on "Plugins" and click remove under the title.
2. Open the profile folder to see if you see a directory with the title. Profiles - Where Firefox stores your bookmarks, passwords and other user data
3. Last resort, export all of the data you want to keep and create a new profile to move the personal data over without it-> Back up and restore information in Firefox profiles

I hope this helps.

guigs2,

Thank you so much! Indeed, "Yahoo Application State Plugin version 1.0.0.7" was there. Since there was no choice to remove/delete it there, I was able to get rid of it by deleting a plugin file which was: npYState.dll

Here is the info and where I found it if anyone needs it in the future: http://techdows.com/2015/02/how-to-remove-yahoo-application-state-plugin-in-firefox.html

Removing Yahoo Application State Plugin in Firefox

1. Visit about:plugins url in the browser

2. Note down the path of the plugin file npYState.dll and close Firefox browser, navigate to ‘C:\Program Files\Yahoo!\Shared ‘and delete it.

3. When you open Firefox and check installed plugins, Application State Plugin won’t be there.

Thanks Again!

Not a problem, however if there are any issues with it, like Hkeys or fragmented plugin files a clean reinstall/refresh might help, just check out how to make a back up of the personal stuff first: Back up and restore information in Firefox profiles