This page could appear for a number of reasons. The Remove a toolbar that has taken over your Firefox search or home page support documentation should have more information about how to remove it from Firefox.

Hope this helps.

I tried but that didn’t work. Thanks

First, please update to Firefox 61 or later. Update Firefox to the latest release . Firefox 52, which you are running, is no longer supported.

Then, please check your home page settings. How to set the home page. It's possible this site accidentally got set as the default home page, so you can set it to whatever you want, or the Firefox default page.

Everyday Manuals (by Mindspark) is a browser hijacker - malware, that you'll of course need to get rid of.

Not only is running all the malware scans that are listed in the link that Wesley Branton gave you, a must - you also need to go to Tools => Add-ons => Extensions, and remove every extension that looks unfamiliar or suspicious.

You'll need to do the same in Windows Control Panel.

See : https://www.pcrisk.com/removal-guides/10408-everydaymanuals-toolbar#a5