I’ve used this before and it seems to work: Firepup650
Raw:
<a class="mention" href="/u/firepup650">Firepup650</a>
Do note that opening in a new tab references the href, vs clicking on the mention itself has different behavior, EX: Firepup650
Raw:
<a class="mention" href="/u/thoka">Firepup650</a>