I'm curious, what is it that makes it Chrome only?

Looks like only CSS transition and animation is special cased for WebKit but that's been unprefixed in Firefox since years back.

https://hacks.mozilla.org/2012/07/aurora-16-is-out/

Edit:

Seems like getComputedStyle is buggy in Firefox?

    « window.getComputedStyle($0).background
    » ""
    « window.getComputedStyle($0).backgroundColor
    » "rgb(74, 144, 226)"
    « window.getComputedStyle($0).backgroundImage
    » "none"

$0 is the currently inspected element, in this case: .js-modal-2

Edit2:

Possibly "works as intended"?:

> getPropertyCSSValue is required to return null for these, and we implement getPropertyValue in terms of getPropertyCSSValue.

https://bugzilla.mozilla.org/show_bug.cgi?id=889091

If so, the fix would be to use the actual attributes rather than the composite attribute.