Hi, is this problem on a Mac (Firefox "desktop" version) or on an iPhone/iPad (Firefox for iOS)?

I think I typically get a pop-up window from PayPal, but maybe there are some sites where the charge is processed in the current tab. Difficult to remember... If Firefox has blocked a pop-up, there usually will be an extra icon in the address bar, either at the left end or the right end, or possibly an additional Infobar below the main toolbar.