There are a number of ways that annoying sites can inject themselves into Firefox, but the most common would be as an extension. This could be something a site recommended you install, or it could have come along with some completely different program as part of a bundle.

You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Command+Shift+a (Windows: Ctrl+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. Then cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious? If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox before the change takes effect. You can complete your work on the tab and click one of the links as the last step.

After removing an unwanted extension, you still may need to restore some settings. Hopefully these articles will help:

• How to set the home page
• Change your default search settings in Firefox

Any improvement?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

When do you get this page?

Did you check the home (startup) page setting in Options/Preferences?