XF 1.5 Safari 9 z-index issue

[Nuno]

Hi.

I'm trying to place in the footer a small "grass" effect using a div.

<div class="bottomHelper"></div>

And the css is:

.bottomGrass {
    background:rgb(108, 170, 245) url('@imagePath/xenforo/footer_bg.png') no-repeat 50% 0;
    position: fixed;
    bottom: 0;
    width: 100%;
    height: 40px;
    z-index: -1;
}

This is what I got when viewing the page with safari.
The background has a very strange behavior.

In the left when I set z-index: -1 and in the right when I remove the z-index.
In the right this div is hover the main forum, so it's not an option.

How am I able to solve this? Is there a trick so I can only show the div to non safari browsers?

Thanks