I just noticed this and I can't quite determine if it's actually a bug or not. I created an HTML email (see this link).
However, I noticed on the formum that several people were saying the templates and their own responsive emails weren't responsive in Firefox. I downloaded the templates and tested my own template (above) and can confirm that Firefox is not responsive on Desktop. If someone clicks view in browser (as someone suggested) while being on Firefox Desktop it is true the site is not responsive. But if the user is own a desktop does that really matter? One would assume (maybe incorrectly) that they are using the full width of the screen to view the email.
However, if you are on an android phone and use FireFox, and click on the view in browser link you do get the responsive version and it works just fine.