Maybe you need to start your OS in safe mode with networking if Windows OS and find that software installed to get rid of it. This is where you should first start and then in safe mode search online for how to remove that search engine as if it's blocking you from removing a safe mode will allow you find and remove it. Otherwise next step is a computer shop to fix this issue.

Hi jonatan, what did the guide have you do?

Hopefully it didn't involve downloading any sketchy programs or giving remote strangers access to your system. If you need a trustworthy malware scanner, see this article: Troubleshoot Firefox issues caused by malware.

Unexpected Search Results Pages

When you are entering a query in the address bar, does Firefox mention using the problem site, or does it mention, for example, searching with Google? In other words, is the problem that your default search setting was changed, or that your searches are hijacked after you enter them?

(1) Default Search Engine

You can change your default search engine on the settings page. You also can remove some search engines from the Search Shortcuts box further down that page. The following article has more info:

Change your default search settings in Firefox

(2) Searches Hijacked to Another Site

Usually, this is accomplished using an add-on with a pleasant name like private search, secure search, anonymous search, etc. But it could be named anything. To investigate, open the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement?

Note: if the Extensions list is blocked or blank, restart Firefox in its Troubleshoot Mode to do the cleanup. See: Diagnose Firefox issues using Troubleshoot Mode.