Hi John,

Nowadays, all the browsers will ask you when extensions want to change your browser settings via a small menu at the bottom. Look for it to save yourself some trouble.

These toolbars will hijack your start page, glue themselves to the top, and barge in as your default search provider.

You can follow the below steps to resolve this toolbar issue:

• First, you need to remove the rogue extension. Head to the three-bar (hamburger) menu in the top right of Firefox, followed by Add-Ons. 
• Choose the Extensions tab on the left, and remove anything related to your toolbar. Banish it with the Remove button. Head to the Plugins tab and choose to never activate the Yahoo (or any other related) entry.
• To fix your homepage, head back to the hamburger menu and choose Options. A popup will come at Firefox’s General tab with the homepage options. 
• Finally, to reset your default search engine click the existing icon in the small search box, then choose to Manage Search Engines. From here you can pick a default and remove any junk ones.

You can take some precautionary measures from next time, so that you don't have unwanted toolbars in your PC. They are mentioned below:

Be Careful Where You Download

Downloading from safe sources will greatly cut down on the junk that your computer accumulates. Stay away from sites likes CNET and Softonic, which often bundle offers in with their software. 

Use Unchecky

Unchecky is an awesome tool that automatically clears the check boxes that get this troublesome software installed in the first place. Grab it from the official website and watch most of your unauthorized installs go away. 

I hope this helps.

Regards,
Lokesh