In which browser is it happening and/or what is the current behavior?
I am currently using Google Chrome as my main browser so this is where it mostly happens but a few months ago, I was using UC Browser as my go-to browser. The problem started there so I thought I can delete its cookies, cache, history, and temporary files and hope it will be fixed but it didn't work out. Then, I tried updating the browser, removing extensions, and finally, deleting it and reinstalling but it was still the same so I abandoned ship and changed to Google Chrome. This leads me to think that it will keep happening regardless of what browser I am using.
I'm not sure if this is still relevant since you already solved the problem of the default browser automatically opening at startup when you asked me to run Malwarebytes but let me share it anyway. Both problems appeared at the same time so I assumed they were caused by the same malware but it was fixed and only the issue of ads opening when I click somewhere on my browser remained.
When I switched to Google Chrome, I did not set it as my default browser so every startup, it opens an ad in Internet Explorer. (Since I deleted my previous default browser, UC Browser, it automatically set Internet Explorer as the default.) This is how I realized that it will open the browser you set as default.
Other than that, the symptoms remain the same as I previously described in my previous post but I dare say that while it is still happening, the frequency of it has been significantly reduced to about 5-8 ads opening in the span of 8 hours.