The 'Clear completed' button doesn't works in FF

[dukex]

The 'Clear completed' button doesn't works in FF(my version is 40.0.3). To works I need to remove visibility hidden and remove pseudo element

diff --git i/app/styles/app.css w/app/styles/app.css
index e2549a6..23906fe 100644
--- i/app/styles/app.css
+++ w/app/styles/app.css
@@ -316,17 +316,17 @@ html .clear-completed:active {
        line-height: 20px;
        text-decoration: none;
        cursor: pointer;
-       visibility: hidden;
        position: relative;
 }

 .clear-completed::after {
-       visibility: visible;
+       /*visibility: visible;
        content: 'Clear completed';
        position: absolute;
        top: 0;
        right: 0;
-       white-space: nowrap;
+       white-space: nowrap;*/
 }

I don't know why is better or necessary a pseudo element, the .clear-completed element has a number of completed task but I cant see it because the element is hidden