Quick hack in userchrome.css:
.CodeMirror-code
{
counter-reset: line;
}
.CodeMirror-line:before
{
counter-increment: line;
content: counter(line);
width: 2rem; /* Adjust if you have lot of lines, big numbers */
display: inline-block;
padding: 0 0.5em;
margin-right: 0.5em;
border-right: 1px solid #DDD;
color: #888
}
Looks a bit strange on long, wrapped lines, but might be usable.
