The first thing that you need to do is locate or create the file usercontent.css in your Mozilla Firefox profile directory. It can usually be located in %appdata%\Mozilla\Firefox\Profiles\\chrome in Windows where xxx is a random number and yyy the name of the profile (default for instance).

It can be found under ~/.mozilla/firefox//chrome in Linux. Create the file userContent.css if it does not exist already and open it in your favorite text editor. If the file exists simply append the code that is of interest at the end of it.

• Always underline links, even visited ones and those that that have no underline because the webmaster used CSS to remove the underline.
  
  :link, :visited {text-decoration: underline ! important };
• Remove Marquee and Blink
  
  marquee { display: none !important; }
blink { text-decoration: none !important; }
  
• Display file type after link. This example is for pdf, you can change it to suit other file types such as .doc, .avi or .zip
  
  a[href$=".pdf"]:after {
font-size: smaller;
content: " [pdf]";}
  
  
• Change cursor for links that open in a new window
  
  :link[target="_blank"]:after, :visited[target="_blank"]:after, :link[target="_new"]:after, :visited[target="_new"]:after {
font-size: smaller;
content: " [new]";}
  
• Display if the link has the nofollow attribute (great for webmasters)
  
  a[rel~="nofollow"] {
border: 1px dashed #f00!important;
background: #f66!important;
}
  
• Increase the minimum height and width of textareas
  
  textarea {
min-width: 50ex !important;
min-height: 12em !important;
}