Why do I have to finish a CAPTCHA?
Completing the CAPTCHA proves you are a human and provides you short-lived access to the web home.
What can I do to prevent this in the future?
If you are on a personal connection, like at home, you can run an anti-virus scan on your device to ensure it is not contaminated with malware.
If you are at a workplace or shared network, you can ask the network administrator to run a scan throughout the network trying to find misconfigured or infected gadgets.
Another method to prevent getting this page in the future is to use Personal privacy Pass. You may require to download version 2.0 now from the Firefox Add-ons Store.