Easy neo! I'm just trying to understand!
See video! https://www.youtube.com/watch?v=L4t_kZGeE3w
As you can see this is FF 26.0. I downloads archives fron RocketTheme and YOOTheme and extract without problem. Problem only with your demo archives!
And I do not use side downloads extention in FF 26 !
And yes, I tried downloads with another br. - Chrome 32.0.1700.76m but archives still broken!
Believe me neo, I would not waste time on this correspondence, if the problem was on my side.