Firefox for Android blocks popups, but not the ones you need to complete the task you're doing on a website. For example, if you click on a link and that is trying to open a popup, it will launch in a new tab instead.
Is there a particular website where you have issues with tabs not opening?
Firefox on Android will block popups by default but when it does, it will show you a message allowing you to open the popups for that site if you want. You can test it by going to http://popuptest.com/ and running any of the tests.