I agree with Lee to check for adware or malware. I am frequently signed in to Google, also use Firefox, and I do not have these pop up problems.
Here is a setting that you should check in Firefox that may help. In the upper right corner click on the three bars ( the hamburger ) and then choose "Settings." Next, at left, choose "Privacy & Security" and then scroll down until you see the section for "Permissions." There is a check box in the permissions section that you can check to block all pop ups or list exceptions to that absolute blocking.
It looks like this: