I’m assuming this is the issue you mentioned in Scroll Video buggy when scrolling up on Firefox - #13 by FoxCreative
Based on the video, it seems like the fixed / sticky position isn’t applied. It works fine on my end using BF 1.0.1 and 1.0.4. My assumption would be that some styles from another plugin are overriding it. If you can share a link to the site we can have a look 