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 Movement Speed Bonus: 25/30/35/40%. Max Spell Amplification Bonus: 20%.
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.