Question

This transition works in Safari & Chrome (= Webkit browsers), but not in Firefox (= Mozilla). Why?

a.lorem {
    width: 100px;
    padding: 20px;
    display: block;
    color: white;
    text-align: center;
    background: rgb(191,210,85);
    background: -moz-linear-gradient(top,  rgb(191,210,85) 0%, rgb(142,185,42) 50%, rgb(114,170,0) 51%, rgb(158,203,45) 100%);
    background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgb(191,210,85)), color-stop(50%,rgb(142,185,42)), color-stop(51%,rgb(114,170,0)), color-stop(100%,rgb(158,203,45)));
    background: -webkit-linear-gradient(top,  rgb(191,210,85) 0%,rgb(142,185,42) 50%,rgb(114,170,0) 51%,rgb(158,203,45) 100%);
    background: -o-linear-gradient(top,  rgb(191,210,85) 0%,rgb(142,185,42) 50%,rgb(114,170,0) 51%,rgb(158,203,45) 100%);
    background: -ms-linear-gradient(top,  rgb(191,210,85) 0%,rgb(142,185,42) 50%,rgb(114,170,0) 51%,rgb(158,203,45) 100%);
    background: linear-gradient(to bottom,  rgb(191,210,85) 0%,rgb(142,185,42) 50%,rgb(114,170,0) 51%,rgb(158,203,45) 100%);
    filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#bfd255', endColorstr='#9ecb2d',GradientType=0 );
    box-shadow: inset 0 -3px 0 #A9A9A9,
                0 3px 0 #EFEFED;
    -webkit-transition: box-shadow .5s linear;
    -moz-transition: box-shadow .5s linear;
    -o-transition: box-shadow .5s linear;
    -ms-transition: box-shadow .5s linear;
    transition: box-shadow .5s linear;
}

a:hover.lorem {
    box-shadow: 0 3px 0 rgba(0, 0, 0, .1),
                inset 0 -3px 0 rgba(0, 0, 0, .1),
                inset 0 0 100px rgba(255, 255,255 , .3);
}

Fiddle

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Answer 1

first, you need to write a.lorem:hover and not a:hover.lorem

after, border-shadow multi values need to be corresponding to their “:hover” pair.
“inset” border-shadow can’t transit to “outset” border-shadow.

exemple :

a.lorem{
  box-shadow: 
      inset 0 -3px 0 #A9A9A9,
            0  3px 0 #EFEFED,
      inset 0  0   0 rgba(0,0,0,0); /* for third ":hover" value */
  transition:box-shadow .5s linear; /* add prefixed verison (-moz-, -webkit-, ...*/
}
a.lorem:hover{
  box-shadow: 
      inset 0 -3px  0     rgba(0, 0, 0, .1),
            0  3px  0     rgba(0, 0, 0, .1),
      inset 0  0    100px rgba(255, 255,255 , .3);
}