Add progress bar styling for Firefox (#360)

I alter this color to reddish for my own site, so it stood out much more easily than the Casper blue not matching the Firefox default. Autoprefixer isn't handling this one. Also, I tried adding this rule, comma separated, to the immediately above webkit rule... but it doesn't actually work when I do that, for some reason. Sorry for the redundancy!

https://developer.mozilla.org/en-US/docs/Web/CSS/::-moz-progress-bar

assets/css/screen.css

@@ -1449,6 +1449,10 @@ Usage (In Ghost editor):
     background-color: var(--blue);
 }
 
+.progress::-moz-progress-bar {
+    background-color: var(--blue);
+}
+
 .progress-container {
     position: absolute;
     top: 0;