[Solved] My class selector is not displayed in rules in Firefox

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your .container class is getting applied. The problem is that your .project class container completely covers the parent container i.e. .container. Add padding to .container class or reduce width of .map class to below 100%, to see it work. You can do it like so – 

.container {
  width: 1400px;
  margin: 0 auto;
  background-color: blue;
  padding: 20px;
}

.project {
  background: red;
  width: 100%;
}
.map {
  width: 90%;
 }

Also its generally a bad idea to hard code height. To make your page responsive, prefer dynamic width i.e. in % for example rather than in pixels.
Hope it helps

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solved My class selector is not displayed in rules in Firefox