Firefox/IE not displaying CSS with Drupal website
It's more like a CSS issue. Your CSS has to be more organize and standard cross browsers. Try not to use "!important", because you don't want to over-use it as you will end up causing headaches down when maintaining the site.

Here are some example you can fix. 
Under file "blocks.css" line 303. 

.pane-block-4, .pane-block-5, .pane-upcoming-events, .pane-block-6 {
  width: 31.25% !important;
  min-height: 500px;
  float: left;
  overflow: hidden;
  position: relative;
}


Since there are 3 classes on each "div"(panel-pane pane-block pane-block-4), you should use something like this instead

#mini-panel-front_blocks .panel-pan {
  width: 31.25%;
  min-height: 500px;
  float: left;
  overflow: hidden;
  position: relative;
}


Hope this help