I’ve had some trouble with the ezPopups Builder in Firefox, sizing buttons, getting the builder to load to 100%, etc. I then remembered that ProfitBuilder worked a lot better in Chrome so I moved to Chrome for ezPopups. Everything in ezPopups seems to work there.
You may also want to head over to the About Firefox section in the browser and see if there is an update. I did, and didn’t see a problem with the builder not loading, or being able to resize buttons.