[Plugin: Good Old Gallery] Flexslider doesn't resize correctly on Firefox

I found Flexslider doesn’t resize correctly on Firefox but it does resize correctly on Opera.

I’m using Ubuntu 12.04 64-bit, Firefox 15.0.1, Opera 12.00, WordPress 3.4.2, Good Old Gallery 2.1, Weaver II theme with flexible width layout. I put the widget in Header Horizontal Widget Area.

https://www.ads-software.com/extend/plugins/good-old-gallery/