I came to it from the web development world, and after getting into speaking transferred internally onto our DevRel team. We're a small team- we encourage any Mozilla employee to speak about their own work if they want, so there's only a few of us for whom it's a full-time job.

Since we're a small team, it's pretty uncommon for a specifically devrel role to open up, but the sibling reply from my teammate callahad is spot-on for how to get involved with Mozilla and Firefox!