Apple support

Popups

1. Click  menu in the menu bar, hold the  shift key on the keyboard down and choose “Force Quit Safari” from the dropdown.

    Safari will quit.

    Launch Safari holding the shift key down.

2. If you might have installed adware or other unwanted software on your Mac

     https://support.apple.com/en-us/HT203987

3. Check for adware.

     Use  the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.

     For instructions:  Install Malwarebytes for Mac          Uninstall Malwarebytes for Mac

     Click the “Scan Now” button. Once done, quit Malwarebytes for Mac.

     Restart the computer and relaunch Safari holding the shift key down.

      Download EtreCheck: https://etrecheck.com/  and post the report here.

      Click, “Free Download” button and then  “Click to download” button,

      open Downloads folder, click on it to open, and then select ”Open”.

      “Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.

Click the "Report" button in the sidebar and read it.

If you want to post the report here:

      Click “Share Report” button in the toolbar and select “Copy report”.

      Click the “Page” button in the toolbar at the bottom of this page and paste it when you reply.

      Note: This is a diagnostic test. 

EtreCheck report will show the presence of third party software that may cause problems.

You are welcome. If you got the free version of Malwarebytes, it will remain installed, but won't run unless you open it and run it.

Malwarebytes was written by Thomas R, a long time poster. It is one of two programs routinely recommended by long time posters. I have run the program on my computer multiple times to test it and have found no problems using it. The program will search for malware/adware. It may be necessary to run more than one scan. After running the scans, restart the computer.

I would try both Malwarebytes and Etrecheck.