Safari

Go step by step and test.

1. Update Safari, if an update is available. Update may come with bug fixes.

    Update to the latest version of Safari - Apple Support

    Update macOS on Mac - Apple Support

2. Force Quit Safari, launch Safari holding the Shift key down.

    About pop-up ads and windows in Safari - Apple Support

3. Startup in Safe Mode. 

   Start up your Mac in safe mode - Apple Support.  

    Quote:

    It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. 

    And it deletes some system caches, including font caches and 

    the kernel cache, which are automatically created again as needed.

4. Clear History

    Clear your browsing history in Safari on Mac - Apple Support

    Please read the article before clearing history, 

    this is like a low level resetting of Safari without affecting passwords and bookmarks.

5. If Safari doesn't open a webpage or isn’t working as expected

    If Safari on Mac doesn't open a webpage or isn’t working as expected - Apple Support

6. Check for adware.

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

       For instructions: Install Malwarebytes for Mac v4      Uninstall Malwarebytes for Mac

       Click the “FREE DOWNLOAD” button.

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

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

       Scan for Malware again.

       Additional steps  if necessary.

  Reset search engine:    Customize searching in Safari on Mac - Apple Support

  Reset  Homepage.   Change your homepage in Safari on Mac - Apple Support