I actually think Firefox is currently seeing more development than Chrome and it's just not as noticeable, because a lot of that development goes into unearthing some of the technical debt that they have (Electrolysis for multi-processor integration, WebExtensions for a proper, stable add-on API), or is an investment into the far-reaching future (Servo).

That being said, I'm not actually that well informed about Chrome's development, so it could also be that I've simply missed some of their development plans...