That is not too difficult. The line number is added via a CSS counter that sets the color of the number to a rather faint #ccc value.

• resource://gre-resources/viewsource.css

pre[id]:before,
span[id]:before {
  content: counter(line) " ";
  counter-increment: line;
  -moz-user-select: none;
  display: inline-block;
  width: 5ch;
  margin: 0 0 0 -5ch;
  text-align: right;
  color: #ccc;
  font-weight: normal;
  font-style: normal;
}

Add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

@-moz-document url-prefix(view-source:){
pre[id]:before, span[id]:before {color: #333 !important}
}

• create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• use a plain text editor like Notepad to create a (new) userContent.css file in the chrome folder (file name is case sensitive)
• paste the code in the userContent.css file in the editor window
• make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userContent.css.
  otherwise Windows may add a hidden .txt file extension and you end up with a not working userContent.css.txt file

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

No joy. I think I followed the instructions (attached css.jpg), but no change in display (ffx.jpg).

Hmm. Works for me. I do notice that you didn't include the !important flag. This flag is essential when you want to override and already defined property.

pre[id]:before, span[id]:before { color: #333 !important; }