Ads page opening on browser startup
It may be an app installed on your phone that is displaying ads while also running in the background. Uninstall any apps you feel display an excessive amount of ads if you don't need them. Also, depending on the phone OS you can go to the recent apps and see a "screenshot" of what process you are doing on the app. if it shows an ad on that screenshot, then that is the app which is doing it.

If your problem is the web browser then uninstall and reinstall those apps again. Do not add any extensions and see how the apps work.

If you are receiving any type of virus or malware then I would recommend using Avast Antivirus or some other virus remover app that checks the contents of your phone and wipes away the virus.

If you've been through these 2 steps, and the problem still persists, you may have to go through and flash your phone with the stock image again. This will remove all your data, but at least you'll know you don't have a virus on your phone anymore.