In all those versions of Firefox this CSS warning is being displayed for Bootstrap 3.3.5 CSS.
They all point to this bit in the Bootstrap CSS.
.progress-bar-striped,.progress-striped .progress-bar{
background-image:-webkit-linear-gradient(45deg,rgba(255,255,255,.15) 25%,transparent 25%,transparent 50%,rgba(255,255,255,.15) 50%,rgba(255,255,255,.15) 75%,transparent 75%,transparent);
background-image:-o-linear-gradient(45deg,rgba(255,255,255,.15) 25%,transparent 25%,transparent 50%,rgba(255,255,255,.15) 50%,rgba(255,255,255,.15) 75%,transparent 75%,transparent);
background-image:linear-gradient(45deg,rgba(255,255,255,.15) 25%,transparent 25%,transparent 50%,rgba(255,255,255,.15) 50%,rgba(255,255,255,.15) 75%,transparent 75%,transparent);
-webkit-background-size:40px 40px;
background-size:40px 40px
}
and later on
.carousel-control.left{
background-image:-webkit-linear-gradient(left,rgba(0,0,0,.5) 0,rgba(0,0,0,.0001) 100%);
background-image:-o-linear-gradient(left,rgba(0,0,0,.5) 0,rgba(0,0,0,.0001) 100%);
background-image:-webkit-gradient(linear,left top,right top,from(rgba(0,0,0,.5)),to(rgba(0,0,0,.0001)));
background-image:linear-gradient(to right,rgba(0,0,0,.5) 0,rgba(0,0,0,.0001) 100%);
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr='#80000000', endColorstr='#00000000', GradientType=1);
background-repeat:repeat-x
}
I managed to remove a few of these warnings by following this method.
http://bytes.com/topic/html-css/answers/723624-css-background-image-error#post2882197
The space between url and the opening { has to be removed for that warning to disappear. I assume it is something similar with the above CSS, though I cannot figure out what to change with it for this type of warning to be resolved. So partly I managed to resolve a few of these.
Then looking at
How to prevent CSS declaration dropped errors cross browser?
Is there a Bootstrap IE specific CSS file or gist? Is there a list of IE specific CSS rules that are needed by Bootstrap?