Firefly also sets Batrider on fire. He's burning away 2% of his max health per second, but gains increasingly bigger bonuses to Movement Speed and Spell Amplification over the duration.
Max bonus values are reached at the end of Firefly's duration and don't linger after. Bonuses increase and self-burn damage are applied every 0.1s. Self-burn damage does not increase over time. Self-burn damage is magical and non-lethal.