GJ Videos is still flash based. Next release will be native youtube iframe or HTML5. Most likely Firefox is missing flash or has an out of date flash. It's built into Chrome it self so tends to always work fine there.
Video fields are already native youtube iframe or HTML5 if you've latest nightly so you can test there if that'll work fine in FireFox for you as well to see if there could be some other issue preset.
Below is the feature ticket for this change.
forge.joomlapolis.com/issues/5873